Re: Yanmar hard starting question
"Also, unspecified additives in a new or overhauled engine could void the warranty."
Not in the US, not since the 1960's.
The US auto manufacturers used to say your warranty on a new car was void unless you used their oil, changed at their dealerships. Someone took them to court and the courts said that was a nice try but also was illegal restraint of trade. Now, if someone wants to claim "You broke it" they have to show specifically what you did that broke it. They can't just say "You used an unauthorized additive" but they have to show specifically, what product did what type of damage.
And any reputable maker of additives will also give you an express warranty that their product will not void the manufacturer's warranty, or they'll pick up the costs themselves.
Which doesn't mean their products aren't of much use for anything except "makes me feel good" and there's always some value in that too.