No reason to replace the shaft until you install 4.56 gears. The only difference is the shaft has a flat section to clear the thicker ring gear. The stock rear cover will work with all the gears. I use Royal Purple synthetic oil. But any synthetic is fine.
Shelby cars tried to tell me I needed to replace the cross shaft if I went to 4:10 or bigger. I told them thats the first I had ever heard of that. Their catalog even states it.
Royal Purple..what weight? Factory weight?
I used the factory weight oil in mine. I lost about 2 MPG on the freeway but my mileage increased around town. I lost roughly 1 MPG city and highway combined. To me, the 4.10's are worth the loss in mileage.
+1!
Mine is the auto with big and heavy 29.2" rear tires and I can still drive quite spirited and get at least 18-19 around town and near 25 on the highway.