It is very unusual for a GMC to have oil in the water. You really don't have any oil galleys near the water jacket. I rebuilt my first 228 over 65 years ago, and I can't ever remember getting oil in the water. Water in the oil was very common. Heads crack, warp allowing head gaskets to fail an cylinders crack. All of this will put coolant in the oil. I would check under the rocker cover. If you have some white slimy stuff you are leaking coolant into the oil and you need to find the cause. If there is no slime under the rocker cover, I would just run it. It might not be a bad idea to put a can of stop leak in the radiator and see what happens.
