Many years ago a close friend had a camero with an engine which was running
very poorly, bogging down, (50,000 miles on it, I think) even after I
verified the timing& advance, ignition, plug wires, etc. was OK. On a lark,
I looked through the oil fill hole into her valve cover. The buildup of
grease-like material must have been 1/2 inch thick on everything inside.
We drained one quart of her oil out and put a quart of MMO in. After 500
miles the valve cover and internals were cleaner, but still a mess. She had
the oil changed and another quart of MMO was added.
Another 500 miles later it was pretty clean, so it was changed out again and
a half quart of MMO added.
After 3,000 miles more miles, the inside of the valve cover looked pretty
clean, minimal crud in the nooks and crannies, so the oil was changed once
more and a splash of MMO added.
She never complained about oil leakage or consumption, but was overjoyed at
having the performance of her car restored. I don't know why the crud in
the engine killed the performance, probably PCV or vacuum related, but it
I didn't monitor the fate of the engine beyond what I wrote here.
BTW - she told me she didn't have the oil changed frequently, which probably
accounted for much of the acumulations.
I, on the other hand, run extended drain intervals with synthetics for the
last 17 years, with pristine, new looking valve train components. I haven't
yet had to open up an engine for a look, so I can't comment on the rest of
the engine internals, but so far, I'm sold on synthetics.
Just remember, this is one test point, and I didn't get to monitor the car
much after the cleanup, so.... YMMV. Sorry I didn't have long term results.