So today I went ahead and had my differential drain and filled. I replaced the old fluid with Mobil 1 Synthetic 75-90. However, the tech at Firestone said that synthetic fluid could damage the gaskets inside of the differential and that it would have been better to replace the fluid with standard differential fluid.

I actually bought the Mobil 1 Synthetic because it has been recommended on this board. Have you guys ever heard anything like this regarding the rear differential fluid?

 

That is a very old thinking....

You really think Mobil would make a fluid that would actually damage any gasket without it being replaced with a brand new one? You know how bad that would look for one of the leading names in synthetic oil?

They had issues with synthetic on older cars like 20 years ago, and back then it was on even OLDER cars.

 

Generally..Yes, Synthetic while being superior will deteriorate/eat natural rubber. I wouldn't worry too much if your seals are New or Healthy but on an aged seal it can cause it to leak. Newer vehicles that call for Synthetic fluids have seals with adequate rubber compound not to reduce their life.
Is OEM full synthetic?

 

**** my boy....this is 95% correct minus that it will eat natural rubber. Being that I work in the industry, the reason why people say that it will cause it to leak is due to syn oil superior cleaning abilities. It cleans all of the crud that's left by years of old dino oil, thus exposing the gaskets that were already ready to leak, but was saved by the crud that was causing a "protection" barrier. Once the crud is cleaned, then the syn oil seeps by the aged gaskets, causing the leak.

 

... pour synthetic fluid on a natural rubber compound and watch it do its Magic, plus the swelling it causes. (another issue on it self)
The cleaning properties..nothing to do with the Natural rubber Synthetic reactions..