This is a real tricky one. I actually kind of just gave up and buy a new grinder every couple years. I actually just bought one today, a SKIL, it was $20 on sale. Cheaper than a tub of some of these speciality greases.
Here's the problem. It's only a semi-enclosed gear box, so you can't just fill it with oil, it'll seep out. So you pack it full of your favourite all purpose grease, something that stays in place. Well, because it's also a very high speed gearbox, so as soon as you turn it on all that grease gets flung to the side and the gears are clean as a whistle.
So you think ok then I'll get a grease with some EP and antiwear additives that bind to the metal while the grease can come to temperature and protect it that way, those usually include some sulfur and phosphates. Except most manufacturers use brass or bronze for one or both of their gears in these things, and EP and antiwear additives will attack the copper in the gears. Ok, so I need just the right viscosity then? Well a lot of these tools never really come to temperature. They're cold for days, then theyre spinning super fast at high loads and high shock 5 seconds later and then they're off again. Can't even get to temperature. Then again, some people use them for a good 5-10min stretch of time and they get very hot, so can use too low a viscosity.
So what about solid additives like some moly grease? That's a good idea, that's why the manufacturers made these gears with powder metal, and impregnated them with lubricant at the factory. How do you know for sure that the lubricant you're adding isn't going to interfere with the self-lubricating system designed into the tool? I have no idea.
It's such a specialized application, to get a grease that meets all the requirements has to be blended specifically for the tool in question, and probably only exists in the barrels that get sent between the blender and factory and to authorized repair shops. Its not available to regular consumers. There are some, fewer every year, tool companies that will sell you the specific grease they say they used in the tool, but now we're approaching the cost of just buying a new one, and they don't send you a tube they send you like a ketchup packet or 2 if you're lucky.
If you're gearbox was just entirely dried up crust with nothing at all ya definitely throw any kind of grease in there, better than nothing, but keep that factory grease for as long as you possibly can.
