What about thermocure? I don't think this has been mentioned in this thread. I used in my 1972 Plymouth Scamp. Worked good, maybe too good. Had a couple small leaks afterward where the sediment had blocked up tiny holes. This stuff cleaned that out. Had to pull the radiator and had it fixed after that. Not expensive to have those holes fixed, and the radiator is in good efficient shape now.
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View my fuel log 2020 Mirage ES 1.2 manual: 42.4 mpg (US) ... 18.0 km/L ... 5.5 L/100 km ... 51.0 mpg (Imp)