sillyboybmxer

just picked up some amsoil 75-140 because i found it locally.
didnt want to wait for the LE to come from across the country, but i am kinda wishing i got it so i can say "my car has only had LE since the first change"...i dunno, i kinda superstitious

neurotic

For those seeking an off the parts shelf solution, why not blend equal parts (i.e. 50/50) M1 75W-90 with 75W-140? The new operating viscosity would be SAE 110 and comparable to the old SAE 90.

slipstream444

Combining two different viscosities will not result in a viscosity somewhere in-between. It's like mixing baseballs and volleyballs together - you don't get softballs. There is likely no major harm in mixing the same brand 75W110 mixed with a 75W140 - but it's not recommended by the manufacturer. Mixing different brand fluids is a bad idea as components in the additive packages of different brands may not play well.
There is possibly some benefit to mixing a single viscosity oil with a multi-viscosity (in hot conditions) if the straight viscosity oil falls in the range of the multi-viscosity (typically at the thick end of the range).

Emil St-Hilaire

75W140 FTW

RedCelica

I used 75w-110...no issues here

neurotic

Really, then what would you get with a 75W-90/75W-140 mixture?

Additionally, how do you think lubricating oils are created?

EDIT:

To further expand the first question, if you mixed fluids with 100 degC viscosities of 15.2 & 24.4 cst why would you expect the mixture's viscosity to be >24.4 or < 15.2 cst?

INDYMAC

I don't intentionally mix oil viscosities. But if you ever change viscosities or brands, it's hard to avoid some mixing. Here is a calculator that you can play around with if you plan to do your own blending/mixing:

http://www.widman.biz/English/Calculators/Mixtures.html

It also wouldn't hurt to ask the company if the products are completely compatible for mixing. Good luck!

ahrmike

Hah! This is an interesting convo, I'll keep an eye out on this thread...

Anecdotal evidence, but for the past 3 or 4 diff fluid changes I have been running Mobil 1 75W-90 LS (couldn't find non LS fluid). Now, every time I change the diff fluid which is roughly once every two oil changes (6-8K?) I notice shavings (sludge, really) on my drain bolt. THIS diff change that I just did today, I ran a 75W-140 Castrol Syntec. I immediately noticed a quieting down on takeoff.. I'll update on the shaving sludge the next diff change.

All changes I have done, the diff fluid came out CLEAR except for the sludge.

zeroptzero

I've been battling the sludge issue for a little while too. When I got the car in 2009 it came with Redline 75w90 in it, no idea of the age of the fluid. I drained it as soon as I got the car and it came out pretty clean with light amount of grey paste on mag. drain bolt. I then ran Amsoil 75w110 for one season (5k miles max) and it drained out with black sludge and quite a bit of black paste on the mag. drain bolt. This year I ran Amsoil 75w140 for one season (5k max) and it also drained out with black looking sludge and black paste on the mag drain bolt. It didn't look any better than the previous run.

I only drive the car in summer weather, very easy occassional driving, not very hard at all. Oddly I sent off a sample of the 75w110 for a used fluid analysis but it got lost in the mail . My sample of the 75w140 from 2011 wasn't overly impressive under a used fluid analysis. I'm going to run an SAE110 LE1605 fluid in 2012 and hopefully be able to compare it to the other fluid at the end of this year.

slipstream444

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.