Blackrider,

I use the below in my FWB300 (Not 'S') and 300SU with complete satisfaction :

RWS R10 Pistol 4.50 (Nominal wt 7.0 gr., actual wt 6.99 gr.)
JSB Match Green 4.50 (Nominal wt 7.3 gr., actual wt 7.32 gr.)
Qiang Yuan Match 4.50 (Nominal wt 8.2 gr., actual wt 7.85 gr.)

You can get the Qiang Yuan from Intershoot or Edenkille.

I also use JSB Express 4.52 (Nominal wt 7.87 gr., actual wt 7.88 gr.) for shooting from 10 to 40 meters.

Have fun

Best regards

Russ