My Mitchell program shows 75w-85 show what you put in should be fine. Were there any metal flakes in the old oil? Could be the syncro's or something more. The bushings on the shift rods could have worn out or bearing could be shot. I'm sure there is more and other's will chime in.

Blacktero: 90 Montero Lwb V6 RS Model. 16" Toyota Alloy Rims, Dual Bouncy's; Gen II Front Rotors & Calipers; Rear LSD

Sraider: 87 Raider 2.6 base. Work yet to be done.