Fractal, this has come up before. B&S’s oil recommendations, the way they are written, suggests there is a massive difference between "synthetic" and "conventional" oils ... and that hasn’t been true for several years. Sure, there is a performance gap between Group I and Group IV base oils when it comes to high temperature operation ... but that's not likely to matter much in relation to what people pick off the store shelves today.
Simply put, it seems that B&S feels their motors run best with sorta, about, approximately a 30 weight-ish oil in
most conditions. So, they’ve taken a dumbed-down approach at writing recommendations. I think it’s possible for those of us who know a lot more about oil than the average operator of a lawn mower can make a better choice.
And the way even some “synthetic” 10W-30 PCMO oils shear down to a 20 weight ... in modest, liquid-cooled applications, is a bit disturbing. I wouldn’t want to use any of those in an air-cooled motor for long ... even though they’d probably tolerate improper lubrication for some time. With these engines, just about any oil will keep them running a fairly long time.
In this thread:
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=14;t=000337
It looks like the only person having any sort of issue using a 15w40 HDEO was
JT who had a problem running a compressor’s motor during a -10F cold snap. He then switched it over to a 0W-30. So, except for cold weather operation, I’ll stick with a 15w40 HDEO ... Schaeffer Supreme 7000 or Chevron Delo400.
Of course, in our brand new 11.5hp Tecumseh, I’ll be using 10W-30 ... Havoline and then Pennzoil High Mileage Vehicle. Once this thing gets a few years on it and is kept in a heated garage, I’ll switch it over to a 15w40 HDEO.
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Bror Jace