on my 2000 400ex it keeps on back fireing. some one told me to change the oil. WHAT oil is best for a 400ex, im riding in very cold weather, Pocono mountains its about 5-40 degrees.

 

Please post a picture of yourself riding at 5 degrees!

I would bet your backfiring is due to the fact that your jetting is off because of the colder weather. Cold air is more dense, so you are running lean.

Your owners manual says 10W-30 oil should be OK from about 10 to 85 degrees F. I would run 5W-30 or 0W-30, below that.

 

Yeah.......Please post a picture of you ridding in 5 degree weather !!!! Are you nuts??? My 400 wouldn't even THINK about starting at 5 degrees !!! But, I guess, some people are more dedicated than others !!! 5 degrees.........Thats just INSANE !!!!!