I have a 1991 Chevrolet K1500 with 4.3 V6. My employer purchased it when it was about 4 years old and about 59k on the odometer. The truck was used around campus, thus very short trips, averaging about 1000 to 1500 miles per year. The oil was changed on average about ever 1500 miles, sometimes less, or about once per year. Mobil bulk oil was used.
On startup, the vehicle had a slight puff of blue smoke. Im guessing leaky valve stem seals. I ran a treatment of Auto-Rx Plus. Vehicle really smokes now, but just on startup. I'm guessing that that the Auto-Rx cleaned some stuff up, thus more smoke.
Some might argue that with such frequent oil changes, the engine should have been clean. But this was a VERY short tripper vehicle. The maintenance records (I have all the records from 1995 forward) show several muffler replacements due to condensation and several starter rebuilds/replacements.
In short, I feel something was probably cleaned in the engine. Im in the "rinse" phase now and will cut the filter open when cycle is completed.