As M1fanatic says, ideally you'll want to re-bias the machine for the tape of your choosing. I don't know what the machines were designed for but it's probably either Ampex/Quantegy 406 or 456 and the chances are pretty good that your machine was set up for either one of those.
Tapes which should work fine (assuming the machine is calibrated for them) include:
Zonal 700, 675, 820 (Canford audio in the UK sell this)
RMGI 911 or SM468 (Available everywhere, also BASF/EMTEC)
Quantegy 456 or 406 (not available new, be careful on ebay, don't use Ampex-branded stuff etc etc.)
It probably won't be able to handle ATR tape or Ampex GP9. Ampex 499 or RMGI SM900 might work, but you'll probably be pushing the record circuits harder than they were designed for.
FYI, most tape manufacturers made tapes that were supposedly bias-compatible with the Ampex standards.
Handy cheat sheet:
Zonal 700 and SM911 by RMGI/BASF/EMTEC are all compatible with Ampex/Quantegy 456
Zonal 820 is compatible with Ampex 406, not sure there's an RMGI equivalent
Zonal 657 is compatible with RMGI SM468 (I think), which is almost but not quite 456-compatible.
RMGI SM900 is compatible with Ampex 499
Quality-wise, RMGI stock can be a bit patchy, they had problems with the early batches shedding like crazy, and unfortunately some of that stuff is still(!) kicking around in the channel. I got bitten by that last week in fact - from observation the stuff that looks slightly red seems to be the bad stuff, the dark brown looking stuff works properly.
