Re: ss/15 transmission
I feel gears should get gear oil too,and my SS-12 already had gear oil in it from whoever else had it before me,so I just top it off with more when it needs any--it leaks a bit from the axles and gear oil oozes out a lot slower!..I see no reason to drain it and go back to oil,if any damage was to be had from using gear oil,its too late now!--and that tranny shifts better and ir quieter than my other 2 Suburbans,that have 30W motor oil in them..
Though motor oil is used even in some truck transfer cases,I doubt it protects as well as gear oil,with it having the "EP" (Extreme Pressure) charecteristics needed for spiral cut hypoid gears in car diffs..some say using thicker gear oil will cause excessive wear due to it failing to flow well enough and reach some bearings,or the drive chain in a transfer case,but you cant prove it by me--most of the trucks I had with a full time NP-203 case had gear oil put in by some cluelless "lube tech",but I never had any of them fail or show any other signs of distress from using it..
I cant see how standard trannys and some transfer cases last ,only having ATF for the lube--seems way too thin and not "slippery" enough to me,to let one last,butsomehow they do....
I'm not recomending you disobey the manual's specs for using 30W motor oil in the tranny--but I think gear oil wont hurt one ,personally..might even prolong its life...wont hurt to add some Lucas to the motor oil--I've added some STP Gear Lube Treatment to one tractor I had and it made it shift easier and whine a lot less..