My family owns a 1997 Lawnboy mower with a commercial grade 6hp B&S engine on it. I'll admit that the oil was not changed every season, and sometimes in its younger days it may have gone 3 years with no oil changes (I was younger at the time and didn't know any better). However, we typically mow 15hrs a season so that's not too bad. Anyway, I've been personally changing the oil consistently for the past few years now every season. Last year I did a Frankenstein mix of Mobil Clean/Pennzoil/Castrol 10w40. It would smoke a little bit on start up so I switched to HD 30. I put Castrol HD 30W in it this time, and it still smokes a little on start up for about 5 seconds...it blows black smoke out of the exhaust. Is this something to be concerned about, or could it possibly be the residual multigrade 10w40 in there getting burned out? Thanks.