My first car was a 138 ci six, which was severely undermaintained, and I used kerosene as an engine flush, to apparently no detriment...Like many boy racers, I modded it with an air cleaner, and thought I was some sort of Genius.
Next car had a 161 ci six, that I over-revved in the pine forests, and built arguably the best engine I've ever done, a 186, 23/59 cam, exhaust, and triple S.U. Carbs (like the one in the photo below). Got very good mileage with the jets leaned out a half turn, but could get ridiculously low when playing.
Next car, had a Holden 202 six, which my brother had flogged, cracking #2 piston skirt (not uncommon), so I built a replacement with 0.060 overbore, high comp hyper eutectic pistons, 28/58, 24/64 cam, V-8 valved head...78,000km in one year, with girlfriends interstate and 150km round trip commute.
A few more Holden sixes, then got my dream car, a Holden XU-1 Torana, came from the factory looking like
Was a disappointement after my home built 186, but if I'd have kept that car, 20 years later, it would have been worth $150k.