Lee has it right. HD oil turns dark. As fot the condition of an engine its not rocket science. Blow by is easy to detect doing a compression test. If there is any significant ring blowby you can hear it at the oil fill and the crankcase vent. Exhaust valve blowby can be heard at the exhaust pipe. Intake at the carb. I did not have a oil filter on a lyc 0320 and within a couple of hrs oil was black.Change oil every 25 hrs it cheaper than an major. For many years we were told to use mineral oil as a breakin oil after overhaul or cyl work today the wisdom don't bother. The lub qualities of HD is better than mineral. Check out the info on shells site regarding breakin HD lub quality .
Just put a new jug on my bird thanks to good ole 100 LL, mechanic put 5 qts of shell HD in. On small continental engines its not unusual to see the front left cyl to go first often the exhaust valve guide and seat . Its out in front and has a tendency to be effected by thermal shock from the flow of cold air. Good reason not to make suddeen power reductions
