I tried Fortes in both my diesel, and Jeans old mazda about two years ago..
The effect on my diesel was not noticeable, but the car was running very well anyway..

Cooking oil has a bigger effect, deeper quieter engine note, and is noticebly quicker, top speed is not effected though..
The effect of Fortes on the petrol 1600 16V fi Mazda was quite dramatic, it immediately cured the rough tickover the car had, (caused by sticky valves I think).. And overal the car performed much better afterwards..

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James. (Nr M67 East of Manchester).
Dark Blue ZX 1.9D Auto 1994 'L' 5 dr (modified) Aura. 98K miles used daily. Ave mpg 40
Wedgewood Blue 75 CTD auto Connoissaur. 2002. 144k. used daily. ave mpg 40 ish.