The VG30DETT will fit in the S30 but apparently it takes a lot of work, and won't get much better mileage than a V8 (My Z32TT averages 17mpg). And if you go with the N/A I don't think you will have more potential than with the V8. Power and gas mileage don't like each other, but a good balance can be achieved. I'm not an expert on V8s, but if I was you instead of doing a swap I would put some money on the V8 you already have to make it more powerful and save some money for gas.