Dirty oil means it's doing its job. Pull your pan and take a gander at everything in there. Maybe it's dirty, maybe it isn't. If you're worried, do Jerry's crankcase flush with diesel fuel in the linked post.
I use the old rule of thumb and change oil every 3000 miles. It's cheap insurance. If you're more comfortable changing your oil every 1500 miles, do so. It isn't going to hurt anything and it costs less than $50 to do it.
https://www.stovebolt.com/ubbthread...4761/54-235-engine-work.html#Post1544761
