I would suggest to not use that type of paper. You could damage everything in printer: paper transport parts, drum unit, fuser... It is up to you...
I understand that it's not designed for thicker card, however I used to run the 300gsm paper through the main tray and it would occassionally get jammed, but when it didn't, it would turn out great (with no line). Through the bypass it runs through fine.
Could it have damaged something when it jammed, and is there anything I could consider replacing to see if it would work?
Maximum weight aloud for 3330 is 163g/m2. So is normal for paper that exceed this weight to have quality issues and printer damage.
I am getting a consisten horizontal line ONLY when printing with thicker card (300 GSM). I only noticed this about 2 months ago. I use the bypass tray for this card, but when I use 200 GSM card, whether in bypass tray or in regular tray, the line doesn't appear. Sometimes it is darker (normally when first using the printer that day) and sometimes it is lighter (after a number of prints), but it's always there, and makes half my page unusable. I've tried checking inside for any leaked ink or anything, cleaning the rollers, etc... any ideas if something could be damaged?
I've attached two examples here - I've coloured out the content I've printed as it's for a client.