I don't have time today to rehash this completely.
For those that don't know me, I see at least one new name, I am a Level II machinery lubrication analyst and I am a Predictive maintenance engineer.
My main focus is lubrication at a nuclear power plant. I also do Ulttrasound, listening for defects in bearings, eletrical gear, etc, vibration analisys, and thermograghy, as soon as my certs get here.
Any way, the weights of gear oil and motor oil are not related. The SAE values are on 2 different scales. Quick example, 20 weight motor oil overlaps the ranges for 75 and 80 gear oils.
For a motor in a HD, use syn, we need it for temp. Syn 3, mobil 1, royal purple, etc. Stick with 20W-50 as recommended.
Use a good filter.
For tranny and primary.
Use the right fluid.
Yes, you can use the same in all, but it cannot and willl not do as good of a job.
It's not the viscocities that are different (much) it's the additives. The anti wear and extreme pressure additives are key.
That said, based on conversations thaqt involved HD factory guys, a Belray guy, and several refinery folk (Love the diverse mix in training claqsses) I am going to Belray for my trannies and primaries at the next changes.
I am planning on using their specific products though. I will look at the hypoid as a choice now.
Belray does not publish their tech data, but they will give it to me if I ask.
So I will do that tomorrow, just made a note for myself.
Search under the oil threads and I posted one discussing this in some depth.  | 
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