so, i have written A LOT now, but conclusion is actually super easy:
there is no magic, just understand how simple it is.
high purity is a must and this is expensive. so stay away from cheap fuel.
keep car fuel system & filters super clean.
keep cans closed and use them up quickly to keep fuel consistent.
after standing still for some time, ALWAYS mix can before filling fuel bottle.
all in all black coated internals and even black built up on the piston is 100% proof of bad quality fuel (castor).
fuel with castor is way more sensible for cold, hot and sun light in terms of storage and longevity.