Harry,
I have and currently still use both.
The 131 is a cleaner/upper cly lube. It does not do anything for increaseing fuel mileage except that if you have any problems with your fuel system like carbon deposits and such, then it will clean and lubricate it which will increase the performance and mileage from the cleaning.
The 131c is like the 131 but for gas only and is suppose to also provide an increase in fuel mileage where the 131 doesn't.
I myself cannot say that I can't tell much if any difference between the two, but remember, I drive like a maniac!. I have been using the 131 and had just recently tried the 131c, so I've been using it myself as well an I'd be hard pressed to say that I can tell much if any difference. Both provide ucl but to say one performs better than the other,
![[I dont know]](/forums/graemlins/dunno.gif)
Jury's still out on that one. I will maintain using either one, what ever I have on hand.
![[Dummy!]](/forums/graemlins/twak.gif)
IMPORTANT NOTICE!*****
Do not use the 131c like the 131. the 131 can be used in any gearbox,hydraulic system,fuel, gas and diesel, and oil. The 131c is ONLY A GAS TREATMENT, not to be confused with the 131.