Building a 49 chevy 5 window truck for my wife for our 30th anniversary, 383 ls1 carbed version. Question is, I'm using a ford 9 inch and it is recommended NOT to use synthetic lube in the rear end? Any known reason why not? Thanks for the replies.

 

Synthetic lubes attack alot of the seals on the market. Anything that uses synthetic oil must use special rubber or fibers for sealing purposes. The recomendation not to use synthetic is probably because the seals used to build the rearend are not compatible. So use synthetic only if you want to ruin your driveway.