If you suspect internal varnish or gum in the engine, look inside the valve cover. If should be clean in there with no brown or yellow or black deposits. If any, I strongly recomend using BG-44 engine cleaner to safely remove the crap. Use it in a good petroleum oil--the 'old fashioned' oil actually will suspend and dissolve lots more crud than the synthetics will. After runnig the BG-44 no more than 2000 miles change oil and filter again, this time using 1 pint of Marvey Mystery Oil in the fill. This will gently soften and remove clear laquer/varnish without causing it to crack and flake off-potentially plugging oil passages. M-1 and other synthetics have been hyped so much that one can easily believe you really never have to change it, in fact just put the container NEXT to the engine and it works magic. Tain't necessarily so as thay say. Premium conventional oil with 5000 mile changes (highway only) or 3000 miles city/suburban short trip use, will keep your engine just as clean or cleaner, will provide just as much wear and corrosion protection, and will remove more harmful dirt ingestion than synthetic oil used for the 10-15K drain intervals touted.

Fuel AND oils, and air filters are much better than 5 years ago at keeping the entire engine clean. Especially if you use 'clean-up' or Teir 1 quality gasolines.
Every 'real world' endurance test has shown conventional oils more economical and equal or better in engine wear protection overall than any synthetic. THe ultra-long drail intervals do not work except under the strict 'long trip moderate or lower load , clean air, high quality fuel regimen.