Triple-H

First off let me say, I do not have a lot of confidence in the V4 search engine AND I have read these
The OIL JOURNALS! Everything you wanted to know about oil but were afraid to ask!
https://www.s2ki.com/forums/index.php?showtopic=195574
Changing from Synthetic oil to Regular
http://forums.s2ki.com/forums/index.php?showtopic=192849

My '90 CRX-Si was run on mineral oil until about 40,000 miles, then I switched to synthetic. It has always had a diet of 10W-30. Now that the car has 90,000 miles and is no longer used on the track I have switched back to mineral oil.

I have heard that switching back to mineral is bad after a synthetic has been used. Can I get a more detailed technical explanation of why?

anarky

I seem to remeber a post by Road Rage on the topic and he stated that there were no problems with switching back to mineral oils after running a synthetic. I, too, had always heard that once you switch to syn you shouldn't go back. I've never bothered on my vehicles once I've made the switch.

Triple-H

The big reason I made the switch is due to the number of miles the CRX-Si gets now days. If I were to use synthetic I would be doing ONE oil/filter change per year, and that just seems wrong somehow. The car is only driven about 5,000 miles a year and I just feel better doing it twice a year instead of only once.

Anybody else?

Dave, you still on vacation??

Where are all the oil experts???

xviper

Doug, I was hoping that Road Rage would see this and give the definitive answer for you. My take is that you can go back to dino oil anytime. It's only going from dino to synthetic on a high mileage car that needs to be done with caution due to the inherent ability for synthetic to find any leak there may be and aggravate the situation.
The only think I can see as something to consider when going from syn to dino on a "well used" engine is that the same weight of dino oil will have more pumping resistance. If you have an old oil pump that might be getting a bit "tired", the slightly higher oil pressure might result in some new symptoms (not necessarily bad).
Additionally, if your car's lubrication system is in "not so tip top" shape, it might be useful to run an engine flush treatment first to make sure all those little oil channels are cleaned out and flowing unimpeded. Dino oil, because of the higher pumping resistance, may also flow less efficiently in those tiny passages.
I think you know your car pretty well and have been maintaining it better than most people, so a flush is most likely not a necessity for you. It's just something for you to think about.

jguerdat

DOUG, YOU IDIOT!! NEVER DO THIS!!!



Actually, I don't think that the issue of synthetic oil "finding a leak" is true. In the bad old days where you'd use a synthetic oil to maintain the same level of lubrication whilst enhancing performance and gas mileage the issue was that you could use a lighter weight oil when switching to synthetic. A less viscous oil would be able to run throuch cracks easier but now we're using the same weight oil all around. The synthetics aren't inherently thinner for the same weight, AFAIK.

Otherwise, I'd agree that there's no reason, other than what RR has posted in the past about folks having bought into the oil companies' advertising about changing oil at 3k miles when the oil itself is quite capable of being run longer, to be worried about going back to dino...

xviper

Where's RR when ya need him?

I am talking oils of the same weights. My "oil upbringing" told me that synthetic of the same weight is actually slightly thinner (thus less pumping resistance) than a dino oil. Certainly, this has been shown by the way the two oils pour at sub-zero temps. Dino 10W30 will flow like molassus at -18*C, while the same synthetic will flow just like when it was at room temp.
I have seen this phenomenon first hand in a couple of my older higher mileage muscle cars. At the age and point where a few leaks developed, having switched to synthetic of the same weight, not only did the leaks get worse, but new leaks formed.

jguerdat

I agree about the cold pourability. However, the issue is at operating temperature. The higher number (10W-30 would mean the 30 weight) is measured at ~200 deg. F (I forget if it's exact or slightly different). If you measure a certain viscosity at that temperature, it doesn't matter whether it's dino or synthetic. The lower number is at 0 deg. F so there may be an issue of pourability figuring in but still, 10 weight is 10 weight.

I certainly don't claim to be a lubrication smartass (Doug would simply use a single word here) so I may be all wet. However, I don't think that there's much room for wiggle here, unlike treadwear ratings of various tires...

Road Rage

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Going from syn to dino can cause leaks - the reason has more to do with deteriorated seals, and the tendency of "old school" dino oils to find those spots and dam them up, so to speak. That is, the oxidized, thick oil factions will attach to those leaky seal points and glom them up.

Synoils have a natural high detergency, and will begin to dissolve those oxidized oil deposits; this will require not only a more frequent OCI for the first few refills, but may indeed lead to a seal leak.

The better GIII dino oils of today are much less likely to do this, and can easily be used for the life of the car. Motocraft's new 10w30 is so good that it is ridiculous for ther price, and I am debating whether I indeed need to use a syn at all.

Going from syn back to min should not cause any problems. Be on the lookout for any increased oil consumption in a high mileage, high wear engine, however. Sometimes the greater film strength of a synoil works in the oil control ring to cylinder wall interface to reduce blowby and oil burning - if you see a sudden rise, ewither rethink your decision, or go to a 10w40 in the summer, and a 5w40 synoil in the winter.