Fluid Type??

I switched to 80w-90 with some Lucas Oil treatment in the original 95,000 4-spd in my 73 Duster340. My tranny was making some noise on hard upshifts and after switching it quieted down. I figure it was either some gear or needle bearing noise and the higher viscosity oil helps compensate for the wear. No problem with shifting either. I would only use ATF in real cold climates.