Metal shavings. What's normal?
 

So I bought a used engine for my 07 450 as the first one woofed the crank. DONT USE 0W40! Deal with the clutch drag. Anyways, there was always some fine pieces of metal in the screen at the bottom of the motor when I changed the oil. The engine I bought is supposedly low hours. It was super clean with no marks on clutch cover and valves in spec so I just bolted it in and got it going without a teardown. Runs like a champ, no strange noises, no smoke. Perfect right? I ran 15w40 dino for the first couple runs. When I changed it, I found, IMHO, excessive metal particles in the screen. My question is this, if this screen was never cleaned before, how much and how large of debris should I expect to find? I'm terrified to ride the thing in case of another failure. The first one was expensive enough and if I have to replace another engine, my wife will not let me have a dirtbikes again.