I use PowerService and use it at "double-dosage" rates at every fill-up.

I have spoken to PowerService several times over the years and they state up to "double-dosing" provides a benefit... but after that, the costs outweight the gains.

No fuel additive will make bad diesel fuel great fuel... but some or double dosing is better than none at all.

I personally year-round and at every fill up. I too agree that a few ounces per fill-up is to little to really help... so I double-dose.