I went out to change the gear oil for the drive motor on my new to me mini excavator, and purple oil came out which looked good and clean. I am guessing it's royal purple, or could it be something else? Seeing this oil made me want to put royal purple back in. My excavator requires a 90W gear oil (GL-4, GL-5) but royal purple only has 75W90 which meets the GL-4, GL-5 spec. Would this be OK to put back in the drive motor? It seems very difficult to find straight 90W oil.
It's not necessarily Royal Purple oil. Manufacturers often dye the oil they install to make locating leaks and determining their source easier.
Mobil fully synthetic gear oil (SHC***) is kinda clear with a purple tint.....
Yes. 75-90 is perfectly fine.
Palmer78, 75W-90 will be fine, and will give you better low temperature performance over a straight 90W. Straight weights are getting harder to find as multi-viscosity oils are here to stay as they provide many benefits and perform as well or better than straight weight oils. At operating temperature both 75W-90 and 90W are the same viscosity. John