I will answer the question. I think the use of synthetic is overrated, I use traditional motor oil. i wouldn't compare aircraft engines and high performance muscle car engines to lawnmower engines. Everything those guys have said is accurate, to include questioning whether you are getting what you pay for in full synthetic oil. I noticed Mobil 1 just had a price drop locally, i thought it was due to the price per barrel of crude.
Keep in mind if you use synthetic you are not suppose to go back and you need to allow the engine break in before using it.
It sounds like you are the type who will take car of your mower. Just by asking the question shows you are on the right track.