I used Tygon fuel resistant F-4040 tubing from McMaster PN 5552K24 with three barbed plastic tees to make a manifold with an oil filled trap at the outlet and it's worked great so far. I cleaned the bowl fitting and the inside of the tubing with acetone and the tubing grabs on to the bowl drains very well. I had used some black plastic tubing with the same results as you (went on tuff but popped off easy) which hardened up after a while and couldn't be reused. Since the new tubing is transparent I can see the oil in the trap and see if anything is going on and it should hold up much better to the gas fumes.(or leaking)