Suregrip additive

I just had my 71 Demon race car's rearend narrowed & rebuilt with 4:56 spool and Mosur axles put in from a respectable local builder. He suggested 75W-110 for it.
For my 8 3/4 posi in my street 71 Demon I run the 75W-90. I use AMSOIL products in all my cars.
TAZZ