Is it ok to do this to my moped?

I have sae 75w-90 synthetic gear lubricant what do you think about putting this into my 1982 general 5 star moped ? If it is ok how much do I put in it I gave 1 quart and it it is not ok what do I use for my moped 1982 geneal 5 star?? Please help me this is the last thing I need to do to I back to riding