As you can see from the TM page that Cranetruck posted, 10 weight oil has several applications.
It can be used as hydraulic oil, engine oil, transmission oil, power steering oil and even more applications.
In the OLD days, we used straight 10 weight oil in Deuce engines when the temperatures were consistantly under the freezing point. When the temperatures were consistantly above freezing, we used straight 30 weight. Nowadays, oil has come a long way and we typically use 15W-40 in deuces, regardless of what the TM says.
We also normally use oil in the 80W-90 range in the deuce transmission, but there are a lot of old transmissions that use straight 10 weight. I have a 10 ton truck with a ZF (German) transmission that requires straight 10 weight oil regardless of temperature. Probably because the manual transmission, transfer case and the torque converter share a common oil bath. That is correct, it is a manual transmission with a torque converter. Weird, huh? It takes some learning to shift it correctly. I buy my straight 10 weight oil from Tractor Supply. Does anyone have another source of supply? If I was in NY, I would buy ALL of what he has, an oil change on my tranny takes 12 gallons!
I used to use 10 weight in my crane hydraulics, but straight tractor hydraulic oil is cheaper in 5 gallon buckets, it works just as well, so now I use that. Tractor hydraulic oil is NOT a suitable substitute for use in an engine or transmission that requires 10 weight engine oil. If I'm wrong on this one, don't be shy, go ahead and contradict me. I may be an old fart, but I'm young enough to learn new tricks!
Believe me, I've learned a hell of a lot reading stuff on Steel Soldiers, it is a fantastic resource.
I trust the posts from a lot of you guys, especially when I know which equipment you own and work on. Except for posts by M813RC, he hasn't got a clue. I'm stoned (legally), that was a joke, forgive me Rory!
M813RC is a VERY good friend!
I'm blathering so it's time for me to shut up.
-Sarge