Once upon a time there was Castrol GTX friction modified oil that engine rebuilders warned no warranty if you'd run your engine in on it coz it would not allow rings to bed in _ I guess in a way they knew or believed it to work then ? eh ?
The line been dropped some time ago.
A few years ago I bought some Wynns

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friction proofing additive not for retail sale, had the lid removed but seal still in place, made in USA.
It was cheap enough to believe in

I used a dose in the last oil change of my EA81 Brumby and only just thought about its inclusion when thinking about economy I have been getting lately
517 km ~ 60.2 litres
517km ~ 60.5 litres
500km ~ 56.6
498km ~ 55.7
my use seems to be so close within 300 to 600 mls difference over same distance for each fill at same bowser all LPG
Maybe it is just better than my other EA81 doin' consistant 540km to 70 litres ??
Friction modifier ??