Taking a break from the >100 ht index outside today. As an engineer always willing to help a fellow engineer, I thought I would offer my opinions on the last volley of thought provoking questions...
- Yes, you MUST use B&S oil in your B&S engine. Use of another oil, such as the Kawasaki oil you mentioned, will guarantee an engine failure before the end of the cutting season. Remember, you should always use the manufacturer labeled oil for all of your OPE and vehicles. Always. Also, use of 'aftermarket brands' such as Quake State, Valvoline, Castrol, etc will void the engine warranty. FYI - B&S is now advertising 'no oil change for life' in some of their new engines. Personally, I think that new philosophy should be back spec'd to earlier models. I recommend you use B&S SAE30 for one and ONLY one oil change. Afterwards, just check oil level at the end of each mowing season and top off as needed.
- As to the air filter....those are not necessary and are just a way for the dealer/manufacturer to pad their pockets. Just think about it....all you have are some nose hairs to prevent potentially damaging particulates from reaching your 'engine' when you breathe. Yet, you are still alive and, I presume, healthy. Right? Aren't you breathing the same air as your engine when you are mowing the yard? If you feel you must have a filter for your mower engine, just cut up some old panty hose and stretch across the intake. An added bonus is that removal of the air filter will result in at least a 40% increase in Hp and torque!
Let me finish with one of the most famous quotes on this site. I never tire of reading it because I always know that when it is stated, the source is obviously extremely knowledgeable and I should just take his/her word for it without asking for an explanation:
"You can thank me later."