I bought a old military bomb loader truck,had a flat head 6 engine.
Can't tell you how many years it sat.
Tires were mostly rotted,gas smelled of varnish.
I didn't clean anything,put 5 gal of gas,a new 6 volt battery.
That thing fired up and pulled out of the dents it settled into.
Stink for a couple of weeks till all the old gas was burnt.
Never invested in additives or newest greatest engine snake oil ever since.
So far everything is fine