John,
I had a problem with my oil cooler leaking. The oil cooler is basically a small radiator that mounts inside the block and is cooled by coolant. When mine leaked coolant passed into the oil, but no oil passed into the coolant. Getting coolant into the oil is a serious problem. It starts attacking the main bearing very quickly. My problem happened over 5k miles or less and I had to replace the main bearings. If coolant is getting into the crankcase, you may see some condensation on the underside of your oil filler cap. Some of the oil and coolant mixture turns to a brown slime. I saw evidence of this at the crankcase vent tube and the uderside of the valve covers. Oil analysis is probably the easiest way to determine if you have this problem. They can check the oil for glycol and water. I hope this is not your problem, but I thought you might be interested in hearing what happened to my motor. The oil cooler repair is relatively cheap, $400 for mine, compared to replacing the main bearings.
Paul