That is true when going from a non detergent oil to a detergent oil.
Phillips XC is an ashless dispersant oil, as are most aviation oils.
Only the straight mineral oil (ie. break-in oil) is not an AD oil. I
don't think avaition oils are considered detergent oils. Marvel
Mystery Oil is effectively a detergent oil and can loosen deposits
that have built up. Word is MMO works if you use it from the start of
an overhaul run to keep valves from sticking. If you put it in an
engine with 1000 hrs on it, you risk the danger of loosening the
deposits that have built up, and possibly blocking small oil
passsages.

Bud


On Jan 28, 3:44 pm, Rip wrote:
Dan Luke wrote:
The oil in my Lyc. O-360 F1A6 is black only two hours after being changed.

What could be causing this? Compressions were in the 70s at annual in
September.Have you changed the TYPE of oil, by any chance? Going from a mineral
based AD oil (like Phillips XC) to a semisynthetic (like Exxon Elite or
Aeroshell), can flush out deposits that have been accumulating for a
long, long time. If so, things will improve with further oil changes,
but you may want to change the oil and filter again soon.

Rip