600 weight? Is that a typo? Normal gear lube thickness is 90, and some of us use 85W-140 synthetic gear lube, but I've never heard of using anything that thick in a transmission! If gear noise is a problem, it might be wise to use an additive like Lucas oil stabilizer along with the gear lube, but I don't think extra-heavy lube is a good idea. For one thing, it consumes a lot of power just to stir up thick lube, and a stovebolt engine doesn't have enough extra power to deal with a lot of parasitic power drain!
Jerry
