I recently bought a 91 GT Auto that just had a complete performance rebuild done to it. Temperatures and getting in the 50s-70s here in South Texas and I was wondering what viscosity oil I should use. I would like to use synthetic. I've also found a reciept in the glove box from the machine shop that did the engine up and it says to use 20W-50 Racing Oil. This doesn't make any sense to me.....dunno why. It just doesn't as I cannot find any Synthetic 20W-50 Oil anywhere. What the bull is racing oil anyway?