Sleeve Bearings...
The type of lubrication in a sleeve bearing matters because of HOW it works.
The sleeve is made of a porous metal (usually Bronze or a bronze alloy). The way it works is that oil is soaked into the pores and creates a microscopic film of oil on the surface of the metal. As the bearing warms up, more oil flows out of the pores in the metal and thickens the layer of oil. During operation, the shaft doesn't actually touch the metallic bearing due to the layer of oil. If kept lubricated properly, the bearing will literally last forever. Most larger motors (think thousands of horsepower) use exclusively sleeve bearings due to the longevity. They actually bathe the entire bearing house in a pool of oil! Some bearings have been around for over a century and are still in operation. Most hydroelectric dams in the northeast have GE gensets which are still running in their original condition with bronze bearings. Those have been turning daily for over 100 years..... I have a Hoover 105 vacuum in my collection from 1919 that's got bronze bearings and still runs like the day it was new! It's been cleaning houses for 90 years!
The problem with grease is that it clogs the pores of the bronze. As soon as the grease gets hot, it becomes a nasty sludgy mess that glazes the bronze. When/if that happens, the bearing is ruined permanently. After the glaze forms, the bearing is no longer lubricated. The shaft will rub on the metal and well.....that's the end of it!
Engine oil is even worse. It contains detergents that are designed to remove impurities and "clean" corrosion or deposits from inside an engine. In the engine, that's what you want it to do! However, when the detergents are in a bronze bearing, they react with the bronze filings that are inherent in the design of the bearing. They in turn actually eat the bronze and turn it into a swiss-cheese like substance. If you've ever used engine oil in a sleeve bearing, pull the shaft out and look what's on it. It'll have a dark black tar-like substance on it. That's the nasty gunk that's come out of the bearing. If you ever use car engine oil in a bronze bearing, you CAN save it if it hasn't been operated for a long time. Soak it in turpentine and get all the traces out of it, then wash it out with Acetone. Soak the whole thing (we're talking FLOOD) with detergent-free oil and let it sit over night. The oil will soak in the bearing and you'll be good to go!
A bearing that's had grease packed in it though is history, never to be saved. If it hasn't been run, you can save it by using the solvent method above, but if the shaft has turned for any time at all, that bearing's days are numbered....
