I just pulled the rocker covers on my 2002 Craftsman Riding Mower/tractor (made by American yard Products AYP). Engine is a B&S 21.5 HP vertical shaft. Engine has 1200 plus hours on it. Never saw Synthetic OCI, except for one Mobil 1 top off. This old girl has had B&S oil factory fill, AAP 20w-50, GTX, Warren convenience store oil, etc. All has met API SL minimum. Last change was Rotella 15W-40. Current fill is Delvac 1300. Filters were either B&S or MotorCraft 400S.
Here is a link of the engine.
Remember, this is an air cooled Briggs Intek V-twin full pressure lube. It sees 25+ hours a month in very dusty conditions.