Are you saying that AFTER you took it for a spin there was oil in the sump, if so how much oil and how far did you ride?
Did you drain the sump before starting her up, and did you fit an in-line one way valve like you were considering, if so are you sure its the correct way around?
I'm sure you are aware that even a few seconds without oil will wreck the motor, so my guess is that if you've ridden it, oil must be being pumped on the supply side - is the oil level in the tank dropping slowly when you run it?