I have found in Florida, living 9 miles from the beaches, the salty air makes my engines require after run oil the same day I come home from flying. Inspect the engine by removing the backplate and see what your bearings looks like. They should have a clean silver shine to them. If I let my engines sit even indoors for a few days, my bearings would be showing rust developing. After run oil is very cheap insurance from a bearing rebuild. Do it after each use, it can never hurt and it's very inexpensive to use.
A whole chapter on maintenance is in a Harry Higley book that's available for $24.95 at most hobby shops, and they state to use after run oil all the time, not just for storage. Good habit to get into and have no regrets. The book suggests to flush the engine as the nitro is the corrosive that damages bearings, regardless of castor content, the nitro is still there. Oils that people have used regularly with success are transmission fluid, Marvel Mystery Oil, air compressor tool oil (I use this as it's cheap from Walmart) and the retail Hobbico type after run oils.
Most 2 stroke .46 sport engines like the AX after break in require a 10X7 prop to properly load the engine keeping it in the rpm range it was designed to run at. I wouldn't use the .40 10X6 after break in is complete, and on a AX it's just 3 tank fulls typically. You can go up to 11 inches and drop one pitch number, so a 11X6 is the correctly prop to use on a trainer as a starting point. Varying factors allow you room to make adjustments such as temperature, altittude, nitro content of the fuel, and weight of the plane among other things. Even choosing APC or Master Airscrew, wood or nylon, makes a rpm difference and the plane behaves differently with these variety of prop materials due to flexibility that causes change in pitch, especially at full speed in the air. You can learn a lot about props from the pylon and combat racers in this forum. You won't be in your trainer long and you'll be off into some other area of the hobby, either pattern, scale, sport, 3D, low wing trainer, biplanes, etc. and each has it's own fuel and prop category to generally follow. But they all typically use after run oil after each flying day.
OS is flexible on nitro content. I have used FAI 0% because it only costs $10 a gallon here. I have used Byrons 5%, Wildcat 10%, Cool Power 15%, and even 30% heli fuel in my OS engines and it runs on all of them. I've changed plugs going hotter and colder when making drastic changes. So 5-10% is fine and common with OS owners for what you're doing. But there isn't anything wrong with other fuels, synthetic or castor based. Some say the castor builds up residue quicker and shellacs the engine too much. Usually I've found by that time, the engine is well seasoned and wiped out anyway. I prefer castor based fuels to help seal the engine, and the castor properties protect the engine better should you have lean run for whatever reason, in an OS ABN engine as yours.