I don't buy that 10 30 oil is inferior at all. All cars use multi grade oil, no exception. If it is inferior, than cars would have broken down. In fact the oil are getting so good now that cars last 200K easily if you change oil often.
All my older cars last 200K and over even using conventional oil from just any gas station by just changing oil and filter every 3000 miles. If you use 30 weight oil in your car, you better worry about cold weather cold start. When the engine is cold, the oil become thicker and don't flow as well and you don't get the protection for the first minute and wear out the engine day after day.
Mower engines are not different from car engines, just keep changing oil and it should be good. In this case, OP might stretched the oil change interval or there is something plugged in the oil path and some parts are not properly lubricated.
As for different brands of oil or even synthetic oil. If one is so much better, then we should see some lawn mower engine last 5000 hour or beyond while the others that use convensional oil last only 2000 hours max. BUT so far, from reading posts, nobody last over 3000 or so. So the question remain whether it is a hype/psychosymetic or fact? Far as for me, I use Mobil 1 10-30 to stretch the oil change interval a little and be done with it, what ever comes......comes!!!