Merry Christmas everyone!
A bit of thread drift, but I was asked...
My choice to not use multi grade oil in my aircraft, is based upon several bad experiences....
When I first bought my C 150, Phillips 20-50XC oil was easily available to me. I found sticking exhaust valves to be a problem (not that O-200's are not known for this anyway). As I worked at an aircraft engine shop at the time, engine teardwn and inspection were not a problem for me. I found that an unpeasant black glass like substance had formed all over the exahaust valve/stem/spring/rocker area. I cleaned it all out, and went again. Months later, same thing... I cleaned it out again.
This time, I switched to Aero Shell 15-50. No more black glass, or sticking valves. During this time, we had two big Continentals come in, both of which had experienced momentary prop runaways in cruise flight. This was accompanied by very low oil pressure indication, but reported normal temps, at cruise power. Both engines went through inspection with no defects found. Both owners changed to straight grade oil, and the problem was never reported again.
Then one day, I was orbiting a friend's place, so he could take pictures. I glanced at the oil pressure, to see with great alarm that it was near zero. Temp normal for a summer day. I pulled the power off, and landed on his runway. When I shut down, and checked the oil, it was the consistancy of a mineral solvent (we call it "Varsol" here). I was very surprised. I left the plane to cool for a few hours.
When I returned late in the afternoon, I checked the oil again, and all was "normal". I fired up for a tem minute flight home. Good oil pressure and temps. By the time I was arriving at home minutes later, the oil pressure was at zero again. After landing check, like Varsol again. The next day, I checked the oil, normal again, fired up, all normal, and flew up to my local airport to buy new oil. By the time I got there (less than ten minutes flight) zero oil pressure again, temps normal. That oil had less than 50 hours and a few months running on it, in an O-200, which does not work oil hard to begin with. Others I spoke with at the timep said they had heard of this too...
Since then, I have chosen straight grade 80 Aero Shell, and never had a problem. As I always preheat in cold weather, I never have a problem starting, and otherwise, with a winter kit when required, I have never had the slightest problem with oil.
I know that multi grades are popular, and to each their own, but I don't see myself using them again, inless the engine specifically requires them.