I have a small horizontal band saw whose gear box should be lubricated with gear oil.

On the other hand, I have a few other old universal motor powered portable power tools that I bought old and used that have gear boxes: angle grinder, straight grinder, and even just drills. I don’t know for these what I should use for lube.

Gear oil that I just put in leaks in my straight grinder (a rarity) so I believe I have to take the oil out. Strange is that when I opened up my straight grinder, the gear box was only moist with an oily substance. Does grease turn into oil after long use?

What to use? Grease or gear oil? Fill to what level when tool is so old that no manufacturer instruction can be obtained? In general for grease, is there a general rule as to how full to pack grease in a gear box?

Finally, can I mix white lithum grease with amber petrolum grease? Can I add both grease and a little gear oil for what I think is better for mixing up?

Thanks