OK ..so I am getting ready to change diff fluids.

tjkj2002 said:

It's the synthetic gear oil,all newer vehicles have about 15k gear oil changes now.

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I thought that Synthetics increases oil change intervals, not decrease then. Can you explain what you mean ?

I'm not being confrontational, just want to understand.

mberthia said:

I thought that Synthetics increases oil change intervals, not decrease then.

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You are correct. Be prepared for tjkj to strongly disagree, however.

mberthia said:

I thought that Synthetics increases oil change intervals, not decrease then. Can you explain what you mean ?

I'm not being confrontational, just want to understand.

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Now this only applies to gear oil,spent 16 hours with oil reps on this,back a few years ago when the federal government mandated a manufacturer mpg average this got them rushing to meet them(will be again as it was raised 10mpg's for 2016).One thing that has always robbed mpg's is weight,and one of the heaviest parts of a SUV like a Jeep and pickups is the diffs,they started reducing weight in the diffs(subpar materials to what they had been using) and started using synthetic gear oil.At 1st they had 100,000mile drain intervals but experienced lots of diff failures.The synthetic gear oil which does have a superior lubricating ability over dino gear oil it does not remove heat as good as dino gear oil.So they started the 12k-15k drain intervals for synthetic gear oil.Most aftermarket diff builders(Currie and Hi9) that make high pinion diffs have also found by multiple failures(and there own testing) that synthetic gear oils just doesn't cut it and strictly advise you use dino gear oil only(call them yourself and ask).

Take a bottle of synthetic 74w90 and a bottle of dino 80w90,put both in the freezer for a couple of hours then see what one pours better.Sure the synthetic will pour better which equals less gear oil sticking to the gears and pinion bearings(very important for HP axles) and why dino gear oil has proven to be better and since it actually conducts the heat better and tranfers it to the housing better then synthetics will it will last longer but at the cost of some mpg's(maybe 1mpg at most in freezing weather).

belvedere said:

You are correct. Be prepared for tjkj to strongly disagree, however.

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Let's hear the comeback.

tjkj2002 said:

Take a bottle of synthetic 74w90 and a bottle of dino 80w90,put both in the freezer for a couple of hours then see what one pours better.Sure the synthetic will pour better

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Well, obviously. 75W (or even your 74W) will have better cold-temp performance than 80W. No one's disputing that.

tjkj2002 said:

Let's hear the comeback

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No comeback, we just disagree, that's all. I was simply telling him what would happen next.

tjkj2002 said:

Let's hear the comeback.

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I know you are but what am I?

Didn't want to join the thread, but it's getting so tempting...

Me too am a syn-freak, but who knows, one learns by living... Had no idea that syn gear oil has a poor heat transfer capability.
I may run some home-made tests one day... ( the opposite of the fridge test.... )

But,even if I take that, why do syns last less, only because of this issue? And, about lubrication, maybe gears DO call for
thicker oil, unlike engines... then I might agree about too thin oil being bad.

But, again, if syns WERE worse, why does Chrysler states one must use 85W/140 SYN in the front diff? Only to rip us off?
(wouldn't be too surprised...)