OldIHC

No doubt this question has come up here before but I could not find anything in a search. What do people use for transmission oil in the old Loadstars? The manual for my 71 Loadstar 1600 with the T17, four speed calls for straight mineral #90 oil. That seems to be non existent around here and I've been using straight #30 motor oil in mine for a while. Seems to work alright but would it be better to use #90 mineral or equivalent?

Available at Tractor supply. I think they call it ford tractor transmission oil.

Straight 30 is fine as long as it doesn't get too hot. 40 or 50 would be closer to GL1-90. SAE 50 is very close to Gl-90, 40 to GL-80 and 30 to GL-75.
The GL-1 has none of the extreme pressure additives that the higher (GL-2 thru 6) do for hypoid gearsets. Transmissions don't need these additives and can even cause problems.
Engine oil is just fine and can handle higher temps than GL-1 without oxidizing.
I run SAE 50 or 40 in all my trans that don't have rows of small needle bearing and require 10w-40. Just about all heavy truck trans will do fine with straight SAE engine oil.
If you haven't been having a problem with the SAE 30, I wouldn't change.

Thanks for the replies. The 30 motor oil has been working ok I was just concerned it might be doing some long term damage if it wasn't mineral oil.

SAE 90, GL-1 Mineral Oil, available at Tractor Supply in two gallon jugs.

In non-syncro trans, all the oil has to do is lube bearings (which do well in anything from grease to diesel fuel) and provide a oil cushion between the gear teeth.
When syncro's are added into the mix, you don't want an oil that is too "tacky" as it impedes the syncro function. This is why EP additives in GL-2 and higher are not recommended.

BTW, there is nothing that causes more disquiet on a board than discussing lubrication.
Only once have I seen wrong lube cause a problem. In most cases, if the item in question has enough lube, it is rare it cause a problem. Lack of enough causes far more problems.
The one case I did see was a truck transmission that had no oil in it, it had been filled with white grease! The gears were just fine, but the bearings between the input and main were toast as were the ball bearing in the case, the grease didn't "flow" into the bearings, and the outer races spun in the case bores.

I always topped it off with 85W-140, maybe that's not a good idea but seemed to give no trouble...Had a 5-speed Clark that called for SAE 50 motor oil, so how much different could that be?....I imagine Case IH Hy-Tran might be a good choice.