zeroptzero

Here's the easiest way to describe what's going on - A 75W110 molecule is essentially a fixed length/size for any given temperature - and a 75W140 molecule is slightly larger for any given point in the operating range (think a helix type of arrangement that expands with temperature) - but the 75W140 being a larger molecule expands to a larger size at the hot end of the operating range (the 75W140 obviously has a larger Viscosity Index - and consequently uses significantly more VIs to give it a broader range than the 75W110 - but it's base oil uses a larger and more dense molecule).
Mixing the two does not magically recombine the molecules into average sized molecules - you just have two different sized molecules mixed together. If the diff sits for any period of time - the two fluids will separate due to their differing densities, at least until things start moving around again.
Try your own experiment - vigorously mix a 75W110 and 75W140 in a mason jar and then let it sit. Even though they are similar in color, you will see them eventually separate (think Black and Tan - just not as dramatic of a color difference). I've done it before - and they do separate over time.

Can this cause a problem? I don't know - but the tech service guys at Amsoil don't recommend it except in very specific instances (you'll have to ask PuddyDad about that one). The biggest issue you can get into is possible foaming - which can be a problem. Once again, if you are using the same brand - I think the likelihood foaming will happen is certainly much less than if you mix different brands.
A diff is not very complex and I don't expect there would be a major problem with mixing a 75W140 with a 75W110 (of the same brand and product line). However, unless you have a specific reason for doing so and have bounced this off the guys that understand the properties of the fluids they make (Amsoil for instance) I don't recommend the practice. Amsoil recommends their SG 75W110 for all S2000s - which is what I use (they list all of their GL5 75W and 80W90 weight oils as well - but the lead gear oil guy at their tech services stated the 75W110 is the best fit for the S2000's diff).

Another point that was brought up by Amsoil Tech Service is that thicker gear oil (75W140 or any of the LE industrial lubes folks are promoting here) is better suited for slow speed/high load applications. If your car never sees highway speeds for more than short periods of time - that may be more appropriate. However, the consequence of a higher viscosity oil is higher temperatures. If you compared two identical diffs, one running 75W110 and the other running 75W140, the diff running the lower viscosity oil will run cooler - especially as speed increases. Under normal driving conditions the higher viscosity oil will run hotter and this will lead to more wear - and likely why JFUSION was seeing increased smudge on the mag plug at drain time. The first fill with 75W110 will not have cleaned all the old redline sludge out - and certainly was only beginning to clean things out. The 75W140 likely increased wear and didn't do much to clean anything out. I would have opted for a short run with a 75W90 to help clean all the buildup out of the diff.

The only time I think I would recommend the 75W140 in an S2000 diff would be if the diff and car were modified and used in high load conditions (1/8 and 1/4 mile drag racing for instance). If this is all your car does - then your diff will likely benefit. If you mostly drive your car in a daily capacity - even if you drive spirited to very aggressively, to include tracking the car - the 75W110 will be the fluid you want to use - and will provide ALL the protection your car will need. If you don't track your car and the diff is OEM, but still drive it with purpose (the way it was meant to be driven) - a 75W90 will work just as well, provide all the protection you need and give you the best mileage.

The bottom line is gear oil viscosities (and types) have a very specific purpose and a higher viscosity does NOT equal less wear, better lubrication or better performance (the "more does not equal better philosophy"). If you run your car mostly on the highway - the last viscosity I would choose would be a 75W140 (or any of the LE offerings - which are non-synthetic gear oils designed for industrial applications). Similarly, a 75W90 is also not optimal for a purpose built S2000 for drag racing.

Lastly (and most importantly) - there is no oil that will protect your car (and its components) from stupidity, neglect and/or abuse.
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Thanks for your summary on that, great info !. I still like the analogy between volleyballs, baseballs, and softballs. Very simple to understand , lol

neurotic

There are too many items here to refute, so I'll make a few quick points:

- There is no such thing as a 75W-90 molecule
- Oils are almost always blends of hydrocarbons of various molecular weights, structures, etc..
- If two molecules have a given set of physical properties, a blend will have physical properties between the individual components (provided no chemistry change/reaction)
- The gear oils should not separate by density if miscible, any details of your experiment?
- LE 1605 has a similar operating viscosity as Amsoil 75W-110 (21.2 & 20.3 cst, respectively), it is only significantly thicker at low temperatures
- LE 1605 is advertised for automotive use

zeroptzero

LE1605 is supposed to be a synthetic fluid now (group 3 or 4 ?), the old LE607 was a non-synthetic fluid though.

INDYMAC

LE 1605 has blended paraffinic 75% amd 25% synthetic base oils according to their product guide and MSDS:

http://www.le-inc.com/msds/1605.pdf

Great stuff!

slipstream444

I don't know about you - but I do have a more than basic understanding regarding chemistry and the chemical/mechanical properties of lubricating oils. I have tried to simplify the basic concept without totally driving the content down below what the least common denominator would understand. With that being said - there's not a hack on this site that can't claim there's a more accurate way to describe a principle or the function of any particular discussion topic - the problem is there's not enough time in the day to go through the exact chemical process, theory or facts to describe "exactly" how something (like a lubricating fluid) "works". I do have a job - and frankly don't have the time or patience to deal with people who want to be contrarian or argue in order to make a point or to protect their ego. There is a reason folks like Road Rage left the forum a while back - and it was centered on individuals who did nothing to contribute to a discussion other than to take points out of context and to niggle over miniscule and unimportant details in order to protect their pride.

For note, I have spent some time discussing the specific properties of synthetic oils with both technical reps and one of the engineers at Amsoil, and I assure you my description is more than adequate for the level of knowledge required for this conversation.

So for you - let me be a little more specific than before.
The BASE oil (not the additive package) is composed of hydrocarbon chains of a fixed length. If the base oil is conventional (derived in petroleum distillation - as LE's offerings are), the oil's properties tend to resemble a linear molecule of an average length - and its viscosity properties will behave accordingly so (fixed viscosity number). The problem with this linear arrangement is two-fold: linear hydrocarbon molecules tend to line up and stick together as they get colder - resulting in the oil rapidly thickening. This also results in the solution becoming less uniform at colder temperatures. This results in poor/marginal lubrication at colder temperatures, and many times results in premature shearing of the oil under high loads at colder temps.

The way this is addressed in conventional oil is with VIs and by the inclusion of different weight base oils (as you mentioned).
The inclusion of different molecular weight oils (however) is also more a function of the fact it is both impractical and too expensive to perfectly filter and distill petroleum-based oil to a perfectly uniform molecular size. VIs allow an oil to maintain its lubricating properties over a much larger temperature range by preventing the oil from prematurely thickening due to its linear molecules essentially sticking together. VIs are chemical compounds that are essentially helical in structure that help prevent linear conventional oil molecules from lining up and sticking together (this is the simple and basic description). The downside of VIs is they tend to be short-lived as they break down faster than the base oil. Additionally, the properties of the conventional oil can be modified with compounds that allow the linear molecule to in essence contract as it gets colder, and expand to its full length at the high end of the oil's operating range - this modification alters the linear nature of the conventional oil molecule allowing it to approximate a partial helix. This can be done with a number of compounds.
Most semi-synthetic oil blends get their "synthetic" credentials from the additive package to include the VIs. The base oil is still mostly conventional (as in the case with LE products)

One of the many benefits of using a pure synthetic oil is the BASE oil molecules are engineered from the ground up (not filtered and distilled from crude) and do not include contaminants that inevitably occur in conventional petroleum-based oils. A synthetic molecule is of uniform size and weight and is designed/engineered with natural branches that both allow it to expand and contract (helix/coil like properties) as part of its natural behavior, and also prevent the oil molecules from sticking together as the fluid cools. This gives the synthetic base oil natural multi-viscosity properties without the use or need for VIs and gives it superior lubricating properties. Synthetic oil molecules are also much more robust than conventional linear hydrocarbon chains and resist shear much better than conventional oil.
Synthetic base oils thicken much less as they cool (Amsoil SG 75W110's pour point is -51F/-46C; SG 75W90 is -54F/-48C) - and more gradually than their conventional peers (LE 1605's pour point is -11F/-12C, which is why there is no Brookfield viscosity listed... not my choice for even marginally cold weather). This is why a QUALITY synthetic will always provide superior lubrication - and do so over a much broader temperature range.

LE 1605 doesn't even come close to Amsoil's low temperature operating characteristics and is even thicker than Amsoil 75W140 at 104F (40C), which would be one of the hottest summer days in Pensacola - and where the kinematic viscosity charts start... 104F (40C).
At this temp, here's the comparison: LE1605: 229cSt; Amsoil 75W140: 193.2cSt; Amsoil 75W110: 146.6cSt.
However, if you live and drive your car in a cold environment and are concerned about cold temperature wear - you want an oil that doesn't start out as a useless blob of molasses - and perhaps does not get to the point where it's fully flowing by the time you get to work. I don't know where you live - but even in the Panhandle of Florida we see temps below the freezing mark on some cold nights. LE 1605 looks to me to be a less than marginal oil in colder temps (does not provide useful lubrication) and doesn't provide any advantage when compared to Amsoil at peak operating temperatures.

The peak temperatures measured in your average S2000's diff get to approximately 150F for ambient temperatures around 50F, and really not much higher than about 175F for the hottest days in the desert South West. In other words - your cars diff does not operate at 100C (212F), where the SAE standards are written.
Nevertheless, here are some comparisons at the 100C benchmark:
LE 1605: 21.2cSt; Amsoil 75W140: 27.5cSt; Amsoil SG75W110: 20.3cSt; Amsoil SG 75W90: 16.9cSt; Amsoil SG SAE 190: 37.6; Amsoil SG SAE 250: 47.4cSt.

Once again - you are free to use whatever gear oil you want. If you have an attachment to LE products or it just makes you 'feel' good to use it - great, you should do that.
I prefer to use facts and logic in my decision making matrix - feelings have nothing to do with why I use Amsoil products. I have thoroughly researched their products (and continue to do so) and will use them for as long as they provide the significant advantages they currently do. Take another hard look at the White Paper I posted earlier and you'll understand why I use Amsoil. It not only measures out as the best overall gear oil in the subject group (ALL tests considered) - it has been proven to be the best overall gear oil you can use in any vehicle you will ever own. LE is boutique gear oil designed and specified for industrial uses - but you can also use it in a car. As such, it's not formulated to cover the broad range of temperatures a typical passenger car will see throughout the year. It is not a bad fluid - but it certainly is not the best fluid for the S2000 - especially if your car sees temperatures anywhere near the freezing mark or below.

INDYMAC

Thanks for the informative post slipstream. I know it took you a long time to write. But it's missing the best ingredient...real world performance in the S2000 diff. Amsoil Severe Gear oils look great on paper, and even in their white paper. But they don't appear to perform that well in use, unlike LE 1605. Since I don't own the UOA's that I've seen, I won't post them. But maybe some others might decide to share what they have.

slipstream444

As I stated in a previous post - I changed my diff oil this past weekend. I sent off the sample this afternoon.
The diff oil was the first fluid installed following the initial ~2,500 mile break-in period. The fluid came out surprisingly clean given there was certainly some break-in still occurring. The mag plug had less of a smudge than I would have expected for the second fill of a freshly built diff - but there was some on there. No chips, flakes or large debris to report - just a smudge resembling graphite. There were approximately 12,000 miles on the fill (my initial intent was to change the fluid out at ~5,000 to clean out any break-in residue - that's what a new baby will do to your memory). The fill was 75W140 (I initially thought it was 75W110 - but the hand pump was on the 75W140 bottle on the shelf - and my S2000 is the only car I own that uses gear oil). I installed a fresh fill of SG 75W110, and will resample at the next 15,000-20,000 mile interval to see how the new diff is wearing.

neurotic

This can likely be considered a personal attack, but this isn't about your, or my feelings. It is about the public dissemination of grossly inaccurate information (not just details).

1) You haven't supported the statements that gear oils of different viscosity/molecular weight are immiscible and will separate
1A) Miscible is miscible, perhaps you observed solids falling out of suspension
2) You haven't supported the statements that oil blends do not have physical properties (viscosity, density, etc...) between those of the blending components aside from a poorly constructed analogy
2A) If you had a very deep ball pit with a mixture of baseballs and volleyballs, would it provide more jumping-into-resistance than a pit of baseballs and less than a pit of volleyballs?
3) You questioned others' oil choice by asking for UOA's, but have provided none to support your claims

Disclaimer: I do use LE 1605, but I do not gather UOA's. I also do not operate the vehicle during the winter/cold. If this was used in an all season vehicle I would use a mutli-viscosity gear oil.

2003AP1

For those of us who only drive in hot climates or only drive during the summer what would be a better option 75-90 or a straight 90?

zeroptzero

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I think straight 90 is getting hard to find these days, plus the SAE90 spec of 10 years ago better resembles the 110 spec today due to classification changes since the S2000 was first built. You might be good running a 75w110 in warm weather conditions, but I think a 75w90 would be fine as well.

I have one UOA of Amsoil 75w140 used during the summer season of 2011 on my S2000, I wansn't overly impressed with the UOA numbers or the amount of paste on the diff drain plug. I'm going to be running LE110 this summer, I'll have a UOA of it used under similar conditions in late fall. Until then who know's what works best, I don't think either fluid is going to cause issues with our diffs, one might work slightly better than the other but the only way to know is to try it and test it. If the damn postal service didn't lose my UOA sample of Amsoil 75w110 I would have better information to share, but it went AWOL. I did see a UOA of the LE1605 fluid on Indymac's S2K which looked very impressive, leading me to try it for at least one run. Im not married to any particular brand though I would say that I have preferred to use Amsoil fluids in my S2K in recent years.