1996 225 Carb/ 2cycle backing into fuel filter

[Noclue1989]

Was down at the boat last night and noticed one of my Racor filters plastic bowls had two cycle mixed in with the gas. I drained it.. started the motor and it refilled with clean gas. What would cause two cycle oil to back down the gas line and end up into the fuel filter?

I do not premix fuel, But did recently switch over to the Pennzoil synthetic blend.

Plan on going down tonight to see if it happens again.

AA