Hello All. I drive my 1600 hf fairly hard, and when going to the Great Northern weekends, it will always include long trips up the motorways, i don't tend to use any additive all the time, and tend to sit at 80-90 on the motorways and have never had to adjust tappets, and have not noticed any drop of power, if i do occasionally decide to use one, i use valvemaster plus, but I too remember something about the 1600hf's having harder seats. Hope that throws a bit more light on the subject.( no pun intended at Italian electrics)
p.s. I used to do night events around here in a 1300s ,and never used an additive, and still beat the local RS 200s.