This! Knurled groove for flux core, u or smooth groove for solid wire. Being knurled has nothing to do with it not wearing out soon. ;)

Thanks for the welcoming. :)
My comment on the roller was in response to the price of it. You will never need to replace it from wear.
My Century only came with the one Knurled roller and that is all that was available for it as far as I know.
The lighter tension means less force on the plastic drive and does not create problems if it is left on indefinitely. I think that was very smart on Centurys part. As compared to a small lincoln ,they used a plastic drive but with a grooved roller. It impressed me so much that I use knurled rollers with solid wire on other machines as well. The tension is kept low and they never slip. The only machine I have used with smooth driver rollers that absolutely never slips ever is a dual drive wheel set up.

You probably know that we do have an introductions forum for new members to introduce themselves if they care to do so.
It is entirely optional; some do & many don't bother but you seem to be rather forthcoming so I thought I'd invite you. :)

Oh, and welcome to SFT. :D