My son has a 1997 Silverado C1500 with a 305 and automatic with 140k miles. It had a problem with heat in the winter, so I looked and found the radiator was not full. I put 1 bottle of Bars Leaks and 1 gallon of antifreeze in it with the engine running and slightly warm. I looked for the leak and it appears to be either the water pump shaft seal or the gaskets between the block and water pump. I checked the dipstick for water in the oil and it was only showing oil on it, no signs of water or anitfreeze in the oil. I did not wipe and re-insert the dipstick to check levels, I just wanted to see if it was showing signs of water. It is winter and I figured we would wait for a warmer stretch of weather to replace the water pump.
Just today we checked the fluids again and first we found the antifreeze was slightly lower than I had left it before. No problem, I figured, since the last time I filled it, it was -2F outside! I filled it and found that there were black blobs in the antifreeze. The antifreeze was also an orange-ish, rusty color, with the black blobs in it. He drove the truck around a little and we checked it again. This time, it was warm and had pressure built in the system. We releived the pressure with the cap and took it off completely. It appears now that the cooling system is filled with black motor oil. It also smells like oil. We checked the motor oil and it was 2.5 quarts low. I am not sure the last time he checked it but it was at least 1 month.
So, it appears the cooling system has a leak which allows motor oil in but not coolant into the oil system. How can this be? I could understand the coolant getting into the oil system from a manifold gasket leak but not the other way around. Has anyone seen a condition like this?