1990 K3500.....Had transfer case case rebuilt 6 months ago....need to know if case requires ATF or gear oil....It is an aluminum case...
Thanks  

All modern chain driven Tcases such as 208 and 241 use ATF fluid. Using gear oil in them can shorten their life. It is curious that you have a 241 though because that model should have had a BW 4470. (2500's used 241 while 3500's used 4470) The drive chain for front axle output shaft is about 20% bigger (thicker and wider) than 241's.  

Thanks SnoMan....I bought the truck brand new in 1990 so that is the unit it came with...We're just getting things back together after an engine swap.....  

It appears then that they went to 4470 sometime after 1990 then. Generally those Tcases are pretty reliable if you service them regularly and use proper axle ratios for tire size in use. Many neglect to change oil in them.