Ok, first don't beat the crap out of the thing, that might mess something up
. Take your drive shaft and put the splined end into the tranny and turn it, while you do that have someone try to put it in gear. If it goes in gear then you may be ok, if it doesn't you may want to be looking to rebuild your tranny.
As for gear oil, yes, 80W90 gear oil will be the weight and oil of choice. I prefer Lucas full synthetics for all my gear oil apps.
Good to hear you are using HKS MHG and ARP hardware!!
Good luck.
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Whats in the garage, well.... in the yard.
1986.5 Toyota Supra - NA - 5 spd
1986 Ford Ranger Lowrider - 5 spd
1984 Ford Ranger XLT - 7" Lift - 4X4 - Auto
"Stuck in the 80's"