you can buy aftermarket filter and oil, I run mobil one syn. oil and filter, there are many aftermarket replacements available
for your starting issue,
both engines or just one?
how long have you owned the boat ?
do you do your own maintenance?
do you know how many hours are on the engines?
how often do you go through 50 gallons of gas ?
When was the last time the spark plugs were changed ?
when was the last time you checked your oil level?
since after a few minutes it eventually starts I would tend to think it's not a cut off switch issue ( your safety lanyard and the swimdeck hatch switch both will cut the spark to the engine) you could easily double check this with a spark tester before you leave out, but both engine having the same problems leads me to think fuel or safety switch,
I would suspect you probably need a spark plug change, the MR1 motors are very sensitive to plugs and often solve a multitude of issues.
A good investment for you is a YDS tool, it allows you to read the engine computers for error codes, engine hours and real time data, it can be found on ebay for about $60 if I remember correctly.
Also, because our boats are big jetskis, the MR1 engine is a common ski engine so anyplace that knows skis should also be able to help you,
if you're not familiar with the cooling system I would suggest watching this video from our member buckbuck,