sliding PEX onto compression fitting

To replace a crimped elbow in my 1/2" polybutylene plumbing, I bought a Flair-It brand elbow at the local hardware store. As I installed it, the elbow 6" away snapped.
(It makes sense. Apparently, stress on the barbs of old-style fittings could cause polybutylene to start to rot. It could also start the old-style plastic fittings rotting. Near those elbows was a broken plastic pipe hanger.)
I bought a second elbow and a length of PEX to go between them. The fittings are supposed to be for polybutylene and PEX, but I couldn't get the PEX on. (The I.D. is 0.5mm or 0.02" smaller.)
The hardware man said heat is used to slide PEX onto these fittings. Rather than explain it, he gave me a piece of polybutylene.
If I have to replace any more tubing, I want to use PEX. How do I slide it on to a compression fitting designed for PEX and polybutylene? Is heat the best way? Would a hair dryer be my best source? Would a different brand of fitting solve the problem? Would faucet grease do the trick?

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