Originally Posted by JonL

There's not a whole lot of force on that line trying to pull it off the barbs. Assuming the ID of the tube is 1/8", the cross-sectional area is then 0.01227 in^2. Let's say you've got 150psi with high viscosity oil on a cold day... the force trying to separate the tube from the barb is only 1.84 lbs.

The proper tubing for the barbed fitting is a very tight fit. It's really hard to get it on the barb, I'd guess that even the minor diameter of the barb is larger than the nominal ID of the tube, so it is really stretched on there.