I've just started using Pre-MIX in my 13B-MSP. Smoother feel almost immediate after the 1st fill-up. I'm putting in bout 200ml (believe it's a US quart) for every 50 litre tank.
Like lennon said RM 60 for a 1 litre bottle to last 5 fill-ups.
As im new here id like to contribute to the silvia forum by offering the original nissan service manual (e-manual) to our s15 silvia members. It contains the recommended feactory service...