The Barth is going in for its spring tune up soon. Since I've owned it, I have never had the rear end oil changed. Should I do this? If so, if synthetic the best route to go?

Thanks,
Rick

That's what I use. I strongly advise against Amsoil - had to have my diffy rebuilt after using it, and MWrench has reporte issues with his, too.

So if not Amsoil, what do I use??

Shell and Mobil both have significant research and application laboratories. Pick your favorite.

I use Valvoline - they make decent products and are widely available. IMHO, Kendall makes the best gear oils, but the application isn't very critical, so most products are more than sufficient.

At the risk of sounding stupid, which my wife says I'm really good at, are the oils you mentioned synthetic or regular?

Other than the short time that I put Amsoil in, I have just used non-synthetic oil.

I also like Kendall and it is not synthetic. It is specifically stated for use in hypoid(sp?) gear systems.

I have just rolled up 208K miles, the whine that was caused by using Amsoil is getting less. I just changed it again before this run to Texas.

I put synthetic (not Amsoil) in the differential last year. The axle made much more noise than usual. (whine) I'm guessing the synthetic was less viscous (75/90) Also the lube got cloudy. (Maybe entrained air or more suspended dirt than usual?)

I went back to the 85/145 conventional gear lube, much quieter than before. Seems to stay clear and less cruddy looking. Evidently my old rear axle likes the thicker lube.

I use 75/140 synthetic - it's the replacement for straight 90.