Sax , old school was break engine in with conventional 30 sae then switch to preferred Synthetic grade of choice for your region temps . Today majority of manufacturers provide syn oil from the factory . I now use Amsoil for all my Engines . May cost a little more but syn oil provision will give you the ultimate engine performance and protection ,

I've used synthetic for awhile in a 12.5 B&S I/C, 5hp B&S I/C, and an 8-1/4hp Wisconsin with no issues, the Wisconsin still has the original 52 year old seals and no leaks using the synthetic. But, since most synthetic oils on the market are just highly possessed petroleum oil I've switched over to regular 15W-40 diesel oil (Mobil Delvac) since it's cheaper and can still handle the heat better.

In my experience in automotive equipment and OPE, almost any synthetic is better than straight 'dino'. A syn blend is a good compromise but using 19 to 20 ozs a year in a mower, you have to go with the best syn you can get. I've gone for several years based on the oil's appearance but in reality you should change it at least annually because of possible grit getting by the filter and acid forming from condensation in the oil. But also in reality, most people don't even change the mower oil at all, much less annually. What we're talking about here is nothing less than 'pampering'. :) Hey, change oil and air filter and the things will last a lifetime. I bought a new mower in 1990 that is still running. I gave it to my wife's sister years ago then it went to my brother who still has it. I pick it up every spring and overhaul it. Its a Homelite-Jacobsen with B&S 3.5 HP, 20" push w/aluminum 'vortex' deck. The absolute best little bagger I've ever seen and it manicures a lawn. I ran 30W Opti-4 in it for some time then started using whatever 30W I could find, like Castrol. It still doesn't smoke. Pretty incredible.

I have a Toro 2007 with a Tecumseh engineand have been using SAE 30 since I inherited it 9 years ago. This spring I switched to full syn 10W-30 and it seems to be doing well.

I used to change the spark plug and now I just inspect/clean it annually and will replace when the electrode edges start rounding out.

Deck is warped as my wife backed the car up into it 8 years ago but it still starts on the first pull, doesn't bog down and cuts like a champ. For the last 4 years I've been saying "Honey, probably gonna need a new mower next year..."

Heh heh I had my Toro 20611 out back in March for some heavy brush wacking. I knocked the oil cap off and it was spraying a mist of oil. I stoppered it with a rag plug and finished the job. Today, I pulled it out and put the repaired dip stick back in. I topped it off with some Castrol 10w40. Some flavor of synth? These days, any oil is good oil. Gassed up the Toro, a 30 something GTS. It started 1st pull.. Later on, I'll drain the Suzuki 4 stroke and sharpen the blade. It will get more of the 10w40