yz250f Maintance

I'll bite. I change my oil about every 5 hours. Fairly hard off road riding not MX.
I put in a clean air filter every time I ride. It's good to have at least 2 or 3 filters.
I bring in a new air filter when it looks like one is going to get a hole in it. They last about a year.
I have a cheap stainless oil filter I clean 80% of the time I do and oil change. It's pretty easy.

Put your clean air filter in after you wash your bike, you don't want to get it wet.

I have the same bike. Bought it new in Nov 05. I change the oil and oil filter every 3 hrs, air filter after every ride. Keep at least 2 air filters on hand. Wash it top to bottom after every ride and check valves every 10 hrs. Still on the original piston but thinking a higher compression piston from a later year and a new cam chain would be cheap insurance and peace of mind. My bike has 55 hrs on it and still looks new. Raced it many times and still runs and starts great. Make sure the valve recall was done if you bought it used. For my age and lack of skill the bike is perfect.

Pretty much what they said, air filter change every ride, keep a couple around so you can just put a fresh one on. Take seat off and cover filter with a plastic walmart sack before each wash. After wash spray anything you don't want to rust with silcone spray.

Use a good oil and change every 3 hrs and oil filter every other time. Buy the cheap Emgo filter, they work just fine and are half the price.

Check valves every 25 hrs and new piston and rings at 50. If your not riding really hard you can prolly stretch out to 40 and 80.

I don't remember a valve issue with the 06 but its been a while since we had one. Very reliable bike, take good care of it and it will last you a long time. Have fun!