WFO thin oil. Cheap as you can get but not drain oil. If your pulling chips there are few fines to worry about. I've cut enough dead ash in the past five years or more to see how my customers, working with their saw next to me, or others I'm watching goo-up the clutch housing and clog air filters in one tank of gas. Funny I don't have that problem cutting the same wood. That is when the file comes out and fines go away. You don't have all day to be clean things when your cutting wood on the clock in the forest or milling down slabs for lumber.
Here is some quality advice for you. We all "thought" we could sharpen a chain in the feild until you worked with a real pro who does nothing but burn fuel and sharpen cutters every tank or two possibly less. Most will stop half way through the second tank if needed and get after those cutters. Once you get this simple fact all the other stuff fades away. I watch these guys goffing around run off three tanks and literally show you how messed up the air filter is and the cutters tells. Go look at Massons chain or any other pro cutter and you will see race chain sharpness or off the grinder sharpness plus2. Ain't lieing man. Until you realize your chain sharpening skills suck compared to these guy you will not improve on round or square. They sharpen cutters by muscle memory, fact. I will never get there based on the fact they do this hundreds of times daily and we don't. It takes me three times as long for the same result. Twice more as long if you get your cutters dull. Four minutes verses sixteen minutes to touchup accurately. You/me/your neighbors sharp chain is dull to them.
Enjoy the pie they will always be serving more
Your theory on thick oil and using less is a fantasy. The wear you imparted to the bar, bar nose, rim and drum bearing wasn't worth the oil savings by dragging the chain around. No need to include the chain that will be stretched to the limits or beyond. It is disposable so once you destroy it it doesn't matter unless it had some cutters still there. The six or ten sharpening you might get is dozens to pros with an endless sharp filed loop. This is wear all your mess is coming from. Chip carries the oil away fines do not.
Stihls are stingy on oil. Anything from this century is the worst. WFO with a good pump and R HO pumps on long bars is the only way to go imho if your not running short bars and large rims. More drivers always requires more oil. 066/660 oil pumps are about done with a 28" bar imho. My 050 36" gets smoked on the mill with a stock 660 oil pump. 404 063 28" is fine. 375 050 28" bucking or milling is also fine. 32" 063 gets dry after ten seconds bucking or felling. You have to lift and wet the bar. 60cc and under are done with a 24" and already starving for oil.
Consider the fact I never run summer bar oil and prefer cannola now on the mill in any weather. Tackafier is all the oil needs for regular tree work. Thin oil moves faster unless you get your saw so hot it boils in the tank like me milling in the summer then it might get messy with a dull loop. Canola boils around 360°F in your tank off the bars studs. 660 R pump can wet a 42" milling on thin oil in warm weather or kept warm. Husky can wet a 50" with a stock pump in comparable 85cc plus saws in any weather. Echo and Mac just dump oil in the rail when adjusted according.
If your using 1-1 gas and oil your likely in the sweet spot to run a long bar. My small saws use half a tank of oil almost wide open. In the winter they run wide open on thin oil and 20" bars 8 pin rim. 7 pin puts out more oil with more rpms in extreme cold and pulls like a truck on those days. Air density adds power but the cold slows down the oil even more if you're not keeping the saw and the oil bottle warm. We don't live in Arizona or Texas so forget about thick oil or starving the bar by turning them down.
