Rear End Gear Lube

[MikeyMan]

A few questions regarding the Gear Lube for the Rear End of a 72 C/10
It's a 12 bolt GM corporate, pretty sure it's NOT a posi. Haven't found the stamped code on the tube yet.
Planning on opening up the cover and confirming the ring gear numbers and ratio, & add new lube.

Can you use limited slip lube in a non-limited slip rear end?
What is the best weight?
I've used 80w-90 on other trucks, but a parts store has 85w-140 on clearance for $10 a gallon.

I'm in Chicago, so we see some very cold and very warm temperature variations. The truck will be a daily driver, nothing heavy duty or Hi-performance.
I see a lot of members singing the praises of synthetic, but not sure that would be necessary in this application.