Synthetic vs Standard gear oil
 

I had my 82 rear-end rebuilt by Bairs and now I'm thinking about using synthetic oil vs conventional. Does anyone have any pros or cons on using synthetic over standard gear oil. Any recommended brands?

I'm one for modifying and using the latest and greatest, BUT, after doing some fairly extensive "research" over the past few weeks prior to filling my rear-end - the standard GM fluid with a bottle of the posi additive is the way to go.

Eaton and the other manufacturers recommend the same, as well as 1 of the 2 resident rebuilder/experts.

AC Delco 10-4061
AC Delco 10-4003

***Note - this is for the pre-80 non-aluminum rear-end. I'm not 100% sure it is the same, but would think so.

I used Amsoil 80W-90 with no additives for years on old, rebuilt and brand new with never an issue. Additives are included

For gear and bearing LIFE, synthetic is the way to go....without question. The only issue might be whether that synthetic lube would have significantly different friction/slip characteristics that cause your [non-slip] differential to behave differently.

63mako's post indicates that is NOT the case with Amsoil 80W-90 synthetic. So, it's your call on this one. If you decide to try the synthetic and it doesn't behave as you expect, just swap it for regular lube. Having synthetic in the 'punkin' will do it no harm.

I have used synthetic Mobil 1 75W-90 for 25 years in the 78 rear end as well with zero issues. No additive needed and the diff performs perfectly. Why in the world anyone subscribes to the belief that a synthetic would be harmful to a diff, gearbox, engine versus a conventional oil after all the years of its proven technology is beyond me. I often read that Eaton recommends GM conventional lube and additive like it did 40+ years ago. The reason is really very simple: Eaton designed and tested the posi diffs 40+ years ago and are NOT going to recommend any different lubricant that they did not test back then. The lack of an Eaton synthetic recommendation does NOT mean that a synthetic is bad.

Agree with Ibanez above. The engineers at Auburn and Eaton both still recommend non-synthetic (conventional) gear oil with the GM additive for their posi's (limited slip carriers) sold new. Of course, the Eaton unit is what was deployed in our C3's.

One must rephrase the question. Just because the engineers at Auburn and Eaton recommend non synthetic does not mean synthetics are not better. I've studied synthetics quite extensively and have 4 years of university level chemistry, and I use synthetics in everything I own. The experts to consult are tribolgists and lubrication engineers. I'd be interested if someone here could quote the recommendation of someone in that field.
BTW I like Amsoil. Mobil synthetics are good, stick with a true synthetic and not a highly refined mineral based oil.

Given that there's no wear or failure associated issues with standard oil and that with the low usage of these cars the change interval is about once per century, what's to be gained by using synthetic?

It's a fair question. There's little doubt about increased performance gains, and improved fuel economy with synthetics. Would it pay for the increased price of the oil? Who knows for sure...

I don't know the recommended change intervals for diff oil, but certainly it would be extended with synthetics.

back in the day we would do gear inspections and sometimes change out gears and the fluid would burn the heck out of you. i changed to synthetic and the fluid under the same conditions would be warm but nowhere near burning you. just the reduction in friction generated heat was benefit enough for me to use it.
i have had good luck with Redline products.

Incorrect! There is a wear and failure rate for every mechanical part. The question is weather synthetics significantly delay that time to failure and wear? The answer has been confirmed over and over as being a strong positive.

As for never changing your rear diff fluid-once per century- that is a disaster waiting to happen especially for conventional diff fluid which will breakdown, regardless of usage, over time. I change my diff fluid every 8 years or so using a synthetic.

100% :iagree: Bingo you hit the mark!

I too have a 4 year college degree in biochemistry (have studied the science for many years as well) and one of my best friends who has a PHD from the University of Chicago in Chemistry has confirmed the same about synthetics. I have used Mobil 1 oil since 1985 in my C3 and today everything I own uses Mobil 1 synthetic as well as you. For gear oil, I believe that Mobil 1 gear oils are true synthetics. Mobil 1 engine oils are not true group IV synthetics EXCEPT Mobil 1 0W-40 European Formula and 5W-50 Porsche Specification-these 2 Mobil 1's are True Synthetics.

The main reason I use Mobil 1 synthetics especially the engine oils is that for 5 qts of a true group IV synthetic like Mobil 1 0W-40 Euro Formula for <$25 for 5Qt container at Walmart (that is less than $5 per qt), there is absolutely no economic argument as to the cost benefit of synthetics at that price point-NONE. As for gear oil, Mobil 1 75W-90 is about $9 per qt. 2 Qts for the diff over 10 years is $1.80 per year for Mobil 1. If I use a conventional gear oil (not GM gear oil which is expensive and the additive is more) at $5 per qt if you can find it at that price point and went 10 years (which you should not) the price is $1.00 per year. So for a savings of .80 cents a year, people are going to squabble over the cost of a synthetic? Makes zero economic sense!

Most of these cars still have their original diff with original parts. The only known wear items are the yokes on later cars and posi units.

The yokes give out due to improper heat treatment, not an oil related issue. The posi units need eventual replacement due to gradual wear of the frictions surfaces, not much different than a clutch on a manual transmission car. There's no evidence that a different oil would slow that wear or that this would not have a negative side effect.

It's wonderful to theorize how much better some oils are but pretty difficult to translate it into real world examples.

Carry on.:thumbs:

A fresh rebuild, go with synthetic all the way!!!:thumbs:
If it was old with old seals I would say stay with conventional oils. I had weeping seals and an outright leaker in a Dana 60 I accidentally used synthetic.

Well the difference between the clutches are the diff clutches are bathed in oil, so the quality of oil may reduce clutch wear. If the tolerances increase due to wear, this results in more traumatic impact between the gears which could result in failure. As most of us have increased horsepower, it's better if the tolerances are nearer ideal spec. If the gear oil lasts 2 times longer than conventional oil, it's easy to choose synthetic for financial reasons also.
I had a '56 chevy 20 years ago, lightly driven, blew the diff gears and the oil was like nasty gravy, so the gear oil does go bad. I think changing oil infrequently is not a thing I'll chance, especially since I spent several grand on my Tom's 12 bolt diff!

And we haven't discussed fuel savings, certainly modest but should more than cover the higher cost of synthetics...

Show me a credible paper proving that synthetic diff fluid reduces fuel consumption.

:iagree:
Synthetic oil is superior in every way that I can think of. The price difference is practically non existent anymore.
If you only grocery get with you Vette do what you want, any gear oil will probably do just fine. And any gear lube might under tough conditions also, but the price difference isn't going to be the deal breaker.
However most of us can't resist WOT at least some of the time. Burn outs, high RPM shifts with a shift kit at WOT etc. A bit tougher on stuff than commuting might be.
Anytime you can reduce friction between meshing parts you are doing them a favor.
Lots of surface area on the gears in a rear diff that mesh. Lots of friction to make heat.
As engines go it is worthwhile to mention the superior flow characteristics of synthetics as well. Equates to oil to parts quicker during start up, particularly on cold days.

I'll do so, from this very thread. 7t9l82 said that switching to synthetic reduced the diff lube temp under identical conditions. Heat reduced is friction reduced, which results in less fuel used. Your turn.

...but do you process warranty claims and replace parts that fail? The manufacturers who do, recommend conventional, non-synthetic, gear oil.