I have heard that about varnish too. Most of it is due to suspended contaminants that stick to surfaces.
Mower still starts, runs, cuts grass and doesn't smoke (unlike me, bad habit, I know).
Still haven't figured out why it uses oil every 8 hours or so. No fouling on plugs, muffler/exhaust has normal carbon, no external leaks.
Maybe I should try some Slime/Fix a Flat in it Darn B&S engines will probably run with roofing tar in the crankcase.
Dave