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?