Hi Stanko, I got a 424 mk3 a couple of weeks ago, and in case you don't have the manual this is what it says:
The Portastudio 424 MKIII is internally adjusted for HIGH BIAS Type ll tape. This means that for best results you should only use tapes of this type. TDK SA, Maxell XL- ll or equivalent formulations are recommended. We strongly suggest that you select one good quality brand and use it exclusively.
As it happens I had a couple of Maxell C-90s left over from back in the day (lol), but I have since got some Ampex High Bias C-60s from here:
Stanley Productions : Audio & Video Products, Copying & Conversion Services
C-60s are preferable because the tape is thicker. You'll also need a tape demagnetizer, a bottle of isopropyl alcohol and some cotton wool buds to keep your tape heads happy!
It doesn't say anything about metal tapes, so I would probably not try them because, as far as I remember, they required a different setting to ferric or chrome ones (and they're not mentioned in the manual)