Any one know who's this is?
I've seen several with pipe fitting and an extended oil fill tube. More oil would have to have some type of effect. I seriously doubt that the chucks trucks get any real milage. In Florida, there is a width restriction that the guys at chucks were telling me about and the big meated trucks they build are past that and not street legal. For us, it way too easy for us to drop in a reliably proven engine that will swing more rpm, pull harder and not allow the truck to slow down as if a down shift on an uphill was made, and OD trannies only spin the NP200 faster past it's military suggested '45mph' top speed. I don't think ANYONE, even Jon has had a problem with them at 45mph. In fact, I think I remember Jon saying that when he'd slow to that the case would cool down. The case just isn't a high speed, and high speed/high torque for higher sustained highway speed unit. It is inherently designed to be a slower speed case. Designed for Early Dodge 6x6's and M37's...how fast could it ever have gone? NOBODY designed the thing to do 60mph. Yes, all gear, real tough...but not a speed demon. Deal with it and replace seals, drive slow or try a set of 4.56's...Here's some figures;
5.87 R&P, 35" tire, 45mph, 1:1 Non overdrive = 2536rpm. So there's a stock baseline for 45mph.
4.56 R&P, 35" tire, 58mph, 1:1 Non overdrive = 2539rpm. 3rpm difference and 13mph gain.
With that the case should happily cruise all day long spinning a near identicle rpm as it stated max speed before.
Just for grins;
5.87 R&P, 35" tire, 58mph, .85% Overdrive = 2778rpm. That's quite a bit more rpm and heat.
I'll say it...if you're planning to not get run off the highway, swap out the NP200 for a later unit. If you're happy in parades, farm use, or cruising under 50 with you stock truck you should be fine with the NP200...it's not a bad case, just kind of became application specific over the years.