Why are you planning to use an additive? Are you having a problem? Frankly few cars need or will benefit from additives.
That said, I would suggest starting by reading the car's manual to see what it may say and second try reading the instructions on the can.
I generally use one bottle of a diesel additive to my VW diesel every year, at the beginning of winter to provide some anti-gel to prevent problems if we get a early cold spell. In over 45 years of driving, that is the only regular use a additives I have used and I have never had a problem that could have been eliminated or reduced by any additive I know of.
Modern cars, oils fuels etc. seldom need special help.