I've tried both Neutra 131 and the SOY 131C in the gas and I can't really detect any difference between the two. They both work well, but the 131C is more than twice as expensive.
Perhaps you could get by with less 131C to help compensate for its higher price, but my gut feeling is that you need at least 5ozs of 131C/10 gallons of fuel.
As far as the Neutra 131 goes, through experimenting and seat-of-your pants feel, I've found that I prefer the higher 10ozs/10 gallons ratio. I add this on each fillup.
Incidentally, this higher 131 ratio corresponds fairly closely to Schaeffers upper recommendation written on their Neutra gallon jugs (I bought a case of four 1-gallon jugs). These bulk instructions tell you to use "one gallon per 100 to 200 gallons of fuel". Using Bob's conversion charts, I find that 1 gallon equates to 128ozs, so I compute the following recommended usage range:
1. Schaeffer's upper recommended usage at a ratio of 100 to 1: This would equate to 1.28ozs/gallon of fuel, or about 13ozs/10 gallons of fuel.
2. Schaeffer's lower recommended usage at a ratio of 200 to 1: This would equate to 0.64ozs/gallon of fuel, or about 6ozs/10 gallons of fuel.
Since these are bulk 1-gallon recommendations, it seems that Schaeffers does not distinguish between a cleaning dose and a maintenance dose.
