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Older wheel bearings (most of which are roller bearings or tapered roller bearings, not ball bearings) do need to have the grease changed out periodically. Newer ones assume that they will be replaced every decade or so; they are sealed to prevent dirt from getting in. And grease is not "lower quality" than oil.
There are few if any ball bearings or roller bearings in any modern engine. Almost all of the ones in there are plain-bearing ones. This is for many reasons, very definitely including cost! Also, roller bearings are (if my understanding is correct) less able to withstand the massive compression loads that the connecting rods put on the rod journals on the crank.
The acids are formed by combustion gases, primarily. A little bit of those gases will go past the piston rings down into the crankcase. (This is "blow-by", and you will likely be seeing a good amount of it in a few more years.) Some of what is in these gases winds up in the oil, though most of it winds up coming out of the PCV (Positive Crankcase Ventilation) system and getting burned in the combustion chambers again. The stuff from the gases builds up in the oil gradually, and some of it forms acids.
Those aren't too good for the insides of your engine, as you might imagine.
Do you have an oil pressure gauge? If not, I would strongly recommend one. You might be surprised at how the oil pressure changes over time.
...Rats, I remember a thread from one forum where they kept one fill of oil in a car (a Camaro, possibly? some big V8 thing) for 20,000 miles or so, and sent a sample off to Blackstone every 1000 or 2000 miles. The oil was plenty worn out at the end, I believe that the author mentioned that when he changed his oil the hydraulic lifters quieted back down again--they had gotten loud gradually, so he hadn't really noticed. I'll try to Google that up some time.
-soD
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