^^^^This…all day long!The T6 will work well. The engine will not self destruct. No cat converter so no problem using "diesel oil".
I mowed grass as a side business for years. Dino diesel 30wt and 15w40 is what I used. All brands. 12.5 to 20 HP engines with no oil cooler. Using Rotella and Delo made me feel good but I soon caught on that the engines ran just the same on Supertech and other house brands too. No big difference. The main thing is to check the oil level at the beginning of each day you use the mower. Keep the air filter clean and change oil when due.
I suspect M1 15w50 possibly Valvoline is what is in the "Vanguard" bottle. 22.99 for a 5 qt jug of M1 is a really good deal plus use that oil for your hydros. BTW Vanguard engines are my favorite. They always have a bit more torque and power compared to the other brands of the same HP. They are hotrods.
Your Scag has a 60 or 72' deck and those are well built, solid machines. Scag makes good stuff. Use the oil that you want, 30/40/50 wt. Syn oil really works well in air cooled engines. Don't over think it.