I use synthetic oil in my 96 Zetec and never use any additives. (oil change once a year, filter at 6 months but only 8,000-10,000 miles per year) However, on the fuel side I do use Amsoil PI (a concentrate) at every tankful and also highly recommend the Chevron
Techron.(Zetec purchased with 30,000 miles now 65,000, keep the PI in the trunk) I have used both and they work great (well, I have never had an injection problem (Camry currently has 140,000 with the PI treatment). Used either of the two on about 7 cars so far and no injection issues. Are they really needed, I honestly don't know! May not have had any problems wihtout using them! I also tried the RedLine Water Wetter in the Zetec due to the higher then normal operating temsp, this had no effect!