Now that I have my rear axle completely reassembled
I need to fill it with oil. I plan to use the 85-140 product. The axle unit will be sitting on jack stands in my unheated garage all winter. Is it better to leave it empty and put it in next spring or fill it up now ?
I'd do it now. Oil prevents rust.
If you leave it empty there's always a risk you'll forget to fill it before starting to run it - and that'll be expensive. The oil won't be damaged by sitting in the axle over the winter.
Do you have modern neoprene seals on the inside of the outer Hyatts?
Do it now and run or rotate the parts to get lube on everything.
Yes, I put in the modern inner oil seals, also the modern outer grease seal. I will put the oil in now and rotate the assembly several times. The drive shaft is installed with the U-Joint on it I can rotate the U-Joint with a wrench easily.