I use 0.6mm wire. I it seems to be working better now that Im using a steel liner, and using a new tip if it starts playing up seems to work for me. when I first used my clarke 151te, I thought I'd use a teflon liner (thinking it would be lower friction) as opposed to the nylon ones I'd used in my Clarke pro mig 90 for 15 years. after a few hours very intermittant use it was feeding horribly, and it turned out that it was the serration put on the wire by the wire rollers was chewing up the inside of the teflon liner. so now I'm sticking with a steel liner (and the wire isnt)
I am going to buy a new liner, so will make sure its a steel one.