Are you sure its a NP535? I "think" that is the same transmission as the NV2500 If so, my 87 dakota had one, 5spd manual. But it used 10w30 motor oil and not synromesh. That came out latter.
I'm positive it's a NP535. My truck as far as I know was originally a V6 3.9 but when I got it somebody put a 318 in it. And now I just put a 360 in it. So I had the trans out putting a new flywheel and clutch in it.
