Most newer hypoid gear oils/EP additives do not attack brass/bronze bushings when I spoke to to their tech department about this, but it is worth checking. I have contacted a few oil manufactures (Amsoil and Valvoline) regarding this. I use Amsoil synthetic hypoid EP geari oil in my lathe head. You can always check the spec sheet "Copper Strip Corrosion ASTM D130". There can be an issue with some gear oils that are specified for limited slip differentials, they have formulations which are slightly stickier to work with wet differential clutch pack. A gear differential oil cannot necessarily be used in a transmission, especially newer cars. Valvoline indicated: "The Valvoline High Performance Gear Oil 80-w90 can be used for both GL4 and GL5. The real concern is the type of sulfate used in the gear oil. You can not use an active sulfate for GL-4 applications. Valvoline uses an inactive sulfate that will not harm any yellow metal such as bronze. This allows our products to be used in manual transmissions that have bronze synchronizers."
I use a Amsoil Gear Lube 75W/80W-90 (AGM) in my gear head and picked up about a 100 RPM on the top end from less friction. Also much less foaming and stays clear much longer. I would expect varnish buildup to be nil.
http://www.autozone.com/greases-and...hetic-gear-oil-sae-75w-140-32-oz-/977483_0_0/
There does seem to be some controversy on the GL-4 and GL-5 specification and stability with yellow metals so I would check with the manufacturer, this article is worth reading. http://www.widman.biz/uploads/Transaxle_oil.pdf
would you suggest the amsoil gear lube for my apron then? it has a number of gears with bronze bushings in it. but everything turning at a relatively slow speed in relation to a gear head lathe