600 W oil / STP

[Bobster]

I am doing some work on the rear axle on my 1928 Roadster. I was going to fill the banjo with 600 W oil, but a fellow restorer claims that STP added to the mix will be beneficial. STP is thick and may not circulate about as well as the 600 W equivalent. I have one source that claims the "modern" 600 W is nothing more than 140 W gear lube. What is the best way to go here. I don't want to "gum" up the works. Thanks, BW