Oil in coolant

I have a 1997 Saturn SL1 that had oil in the coolant overflow tank. The oil level in the crankcase was fine so I had the coolant system flushed and refilled, now about 100 miles later I have oil in the coolant and no oil in the crankcase. Can someone tell what is wrong with car and give me some idea what it might cost to fix.

These engines were known for blown headgaskets. Fill the fluids back up and run the engine with the radiator cap off. If you see bubbles, that’ll be combustion gasses being blown through the breech in the headgasket and migrating up out the coolant fill hole, the highest point in the system.

If you want to be certain, do a pressure leakdown test on the individual cylinders. It’s wasy to do, the kit is cheap, and the instructions come with the kit.

The headgasket not only seals the top of the cylinders, it also seals the coolant and oil passages from the block to the valvetrain. If it blows a hole between passages, the fluids can mix.

NO oil in the crankcase? after 100 miles, I’d say that would fry most engines. do you mean you have no coolant in the crankcase, and if so how do you know?
Oil in the coolant, what kind of oil? If you have an automatic transmission, and it has cooling lines in the radiator, if those lines cracked, it would put transmission fluid in the coolant.
If you have a blown head gasket that is allowing communication between the oil galleries, and the coolant galleries in your engiine, then what usually happens is the coolant gets into the engine oil, but sometimes oil will get into the coolant, or each fluid will contaminate the other. In this case, after running the oil will take on a milky color and get frothy too. Now the same can happen with a cracked head or a cracked engine block, but usually its the head gasket that is most suspect. to fine out, you need a compression test. No telling what it might cost until you have determined just what is broke.

If it was trans fluid it would have lunched the trans very quickly. The cooling circuit tends to be lower pressure, but could match or exceed coolant pressure while operating. That would go out the window as soon as he shut the engine off. The pressure in the rad would infuse coolant into the cooling lines. Very quick death for a trans.

Getting oil in the coolant is pretty rare. It’s usually the other way around. I’ve only seen it once (not my project/job). The same rules tend to apply. Once you shut the engine off, the coolant pressure trumps ZERO oil pressure. While the head gasket would appear to be the cause, the one engine I saw with this condition was not fixed with it replacement.