Had a '60 Chevy 235 and I would change the oil and within 100 miles the oil was black. Stuck a stick in the engine and found a sea of muck in the pan. Something I tried and it worked great. Get the engine hot. Drain the oil and fill the crankcase to the full mark on the dipstick with a 50/50 mix of fuel oil and transmission fluid. Run for 2-3 minutes and drain. Only let the engine idle! Don't drive it! On the 2nd flush, I had the valve cover off and poured the mixture over the rockers while the engine was running. After about 3-4 flushes the stuff started staying clean when I drained the pan. Gave it an extra shot for good measure. Problem solved.
