A TN-6300 is supposed to last 3,000 pages. This one made it to 3670. At the end it was still struggling and intermittently outputting pages.

Now replaced with a TN-6600, which is listed as 6,000 pages. Might refil the old one and reuse it.

Anyone know if there is a capacity difference between the two? TBH they look the same. If I can just refill it to double using a toner kit it would save me a lot of dosh.

I wouldnt bother.
The drum unit inside the Toner will probably fail befoe the toner is used up.
Unless the drum is seperate from the toner

Cartridge world claim to replace all the inners when they refill toners, they also refill them to the same as the higher capacity carts.

IIRC the TN-6600 lasts twice as long simply because it has twice the toner, normally you can refill  two or three times without problem.

The Drum is a separate entity on mine. An official one costs more than the price of a new printer   but you can buy remanufactured ones to suit at a quarter of that price, they are supposed to last 20,000 pages.

with my Sammy ML1510, the original cart easily took the full amount of toner (in my case a 1000 page cart happily took 3000 pages worth). The drum is integral in the cart with the Sammy printers and the original cart is now getting a bit ropey, but its still very useful for draft printing and large print jobs of things I dont much care about the quality of but just want hardcopy.

Most of my output is book manuscripts for editing, I print very little that goes to other people so it doesnt matter if the print looks a little ropey providing it is readable.