White soot on exhaust

While taking my new to me'62 Cessna 172 with an O-300 (with about 75
hours on it SMOH with Millenium cylinders), I decided to start leaning
as I should on my last flight.
During the next preflight I noticed a whitish soot around the exhaust
and on the cowl just adjacent to the stack during the preflight.
Looking up on r.a.o for "white exhaust soot" I came across posts from
several years ago about the white soot being a side effect of leaning.

Does this sound right?
Am I leaning too much? (I'm still trying to figure out the lean and
rich of peak thing, especially for this particular airplane that I
have about 18 hours in).

Thanks,
Ryan Wubben
Madison, WI