Posted by Greg Coffin [172.68.35.114] on Wednesday, March 26, 2025 at 16:10:57 :
In Reply to: WM300 danish model Transmission fluid posted by Black Eight Motors Germany [162.158.172.72] on Thursday, March 20, 2025 at 04:18:29 :
I like to use 85-140 multigrade standard oil (non-synthetic) in my trucks. The 1968 trucks didn't originally have any brass in the transfer case or differentials, so you can use any of the standard/modern oils, which often have additives that will attack brass and bronze.
The NP420 transmission has a bronze bushing in the reverse idler gear, so it is best to use what is referred to here as a GL4 or 'brass safe' lubricant. I believe the synchronizers have always been aluminum, but check that.
There are rebuild kits for the differential cases that have brass shims to replace the original fiber washers for the spider gears. If the differentials have these brass washers then they too should use the GL4 or 'brass safe' oils.
If the truck has a winch on it, the bronze bushings need the 'brass safe' oil as well.
Best of luck with the project!