Hello. I own a well used (6000 hrs?) Pel Job EB 12.4 1,3 ton mini digger. It have a small leak in a valve, but so far, I've found it better to just add some oil from time to time, than overhauling the valves. I let it out, and I always tell the users there are a small leak, but not to worry. The last one did worry, and bought a new 4-liter (1 gallon) of 80W90 GEAR oil, adding it to the tank. Will this do any harm in any way? The total system is around 30 liters, I think. Do I have to change all the oil?
Yes, and sooner rather than later. There are some nasty EP additives in the 80W90 gear oil that should not be in contact with certain hydraulic system seal parts longer than is absolutely necessary.
I was afraid of that Is it enough to empty the tank and fill fresh oil on it, or shuld I flush it, meaning I change the oil two times?
I'd change it at least a couple of times to be sure, just changing the oil in the tank would not be sufficient.