Ok, I will be taking my car to finally get the clutch and rear done with both hardened output shafts. I figured while having this done, I will have the tranny fluid changed as well as an oil change, as I have 20K miles and Im due. I spoke to the shop to ask them about using Amsoil, and they told me that they use the stock GM fluids and Mobil 1 oil. This is a very reputable shop, however, should I insist on using Amsoil in the rear and tranny? Since this will be a new rear, is it ok to use Amsoil right away? The shop said that the GM diff fluid is synthetic and thats why they use that. Also, how much should I expect to pay for the tranny fluid change if they are using the stock GM fluid, and how much more should I expect to pay to use the Amsoil? Please let me know since everything will already be apart, I want to do it right the first time. Thanks in advance.