[rotorwrench]

It not ISO-1500 so it's not as viscous but it is ISO-1000 so it's still pretty thick stuff. Lincoln used to use SAE-250 for the gearbox & axle assemblies so it's not unheard of. You can run GL5 in the transmissions for a while without eating any bearings up but its the long game that can cause problems eventually. If the ISO-1500 is working then use it up unless you bought a 55 gallon drum of the stuff.

I am still using up and old pail of Texaco Thuban SAE-140 that I use for build up oil in aircraft engine overhauls and that stuff is pretty thick. It's definitely noticeably thicker than SAE-85W/140 multi vis that I use in the helicopter gearboxes. SAE-250 is too thick for my old Mercury overdrive transmissions. I use the Stay-Lube 85W/90 GL-4 for them. It's a mild EP but those transmissions have a one way sprag free wheel unit in them due to the overdrive. They can't take high EP at all. Sometimes I just use SAE-50 grade 100 aircraft motor oil in them but it has to be the oil with no TCP lycoming additive in it. The TCP is worse than the sulfur/phosphate EP additives in gear oil. SAE 50 motor oil is the same viscosity as SAE-90 gear lube since they are graded differently.