I've got a 1999 Dodge Ram 3500 4x4 DRW with a Dana 60 and 4.10 gears. I put a 4x4 posi-lok on it last night so I can actually use 4 wheel drive and lost some fluid, and I'm finding conflicting reports of what I should replace it with.

Dodge Ram Fluid capacity and specification Information

That says 75w-140 synthetic only (WITHOUT friction modifier? or with?), 8.5pts capacity. Advanced auto part's computer says it takes regular 80w-90, but again, couldn't say whether I'm supposed to use friction modifier or not, and they said 7.4 pts capacity.

Depending on the cost and the amount I have to use, I might drain it and replace it all, or just top it off to replace what I lost.

Any insight here?