K, what model? I guess 240 by your nick?

I've used dino and synthetic. I had one limited slip diff take a dump on me, and it was using dino oil. But that is not an implied warranty that using dino oil in a limited slip diff WILL BE THE CAUSE of catastrophic failure. It will not.

I really dig synthetic for the automatic locking differentials (with limited slip additive) and it makes a nice difference, I've noticed, with respect to ALD function. Especially when the conditions are very icy, and the little bit of added delay in engagement keeps the back end from flying out when it engages, vs. gently coming out of line.

But the ALD supposedly does OK in lighter horsepower applications with normal fluid as well. In fact, it should make little difference with respect to longevity, unless someone is really pouring the steam on with tires on dry pavement. Then it might shorten the life a little bit. Also in applications like Tahoes or something that does a lot of towing AND has a lot of HP, AND is sometimes operated in off road situations, the ALD would be much better off with synthetic fluid and friction modifier (LSD additive that is built into most synthetic fluids apparently). The 4WD crowd is pretty pumped about the Eaton unit that Volvo uses (as do many other manufacturers) and they seem to favor the synthetic fluids. I think that GM might be using their own synthetic fluid for those units, as they had some failures. That's just off the top of my head, though.

If we're talking 240s, though, you probably have just the standard open diff. They last literally forever as long as there is fluid in them. I think that following a 25k or 50k diff service would practically guarantee you that it would be there until you are sick of the car.

Someone mentioned that the car is a little bit less tight on cold mornings with all synthetic fluid, and I have to say, that is very true. Both trans and rear axle fluid being synthetic, make a big difference.

The choice, basically, is yours. Sure, synthetic is great... but dino isn't bad either.

That is said only with what I know about the axles I've spoken about... and I'm not a rear axle guru.
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1992 940 wagon, low miles
as well as others.