My 2009 Gen 2 Prius warned against it, but I do not find a warning in the Prius c manual. If you HAD to, I would say every 100,000 miles.
I use a bottle of Techron with a full tank of gas in mine at every oil change, I change oil at 15,000 mile intervals. It seems to work well and many times the auto parts store has it buy 1 get 1 free.
Old, but worthwhile article regarding fuel injection cleaning. Wish I read it before agreeing to Honda service rep selling me this service on my old '08 Civic. Injector Flush: Are They Taking You to Cleaners? - Los Angeles Times "Fuel-injection cleaning actually makes less sense than it did about 12 years ago, when the injection systems started to replace carburetors. At that time, poorly formulated gasoline and badly designed injector nozzles caused a lot of fouled and plugged injector tips. But that's far less the case today."
I walk by a Mr. Lube afternoons weekdays, and occasionally they've got a customer out front with the hood up and an intravenous bottle hanging, tube running to the air intake I guess. It used to be you had to hold your breath as you walked by: there was a pretty rich cloud of hydrocarbons... Of late not so bad, maybe the stuff they're using now is more benign? Anyway, never considered it, various cars over the years don't seem to have missed it.
I use about one ounce of Lucas oil stabilizer for every five gallons of gas. This is used as an upper cyclinder lubricant, to lubricate the fuel pump (has anyone ever heard of a fuel pump going on a gen 2??), it also cleans fuel injectors, suppose to make for a more complete combustion, cleaner spark plugs. It's cheap if you buy the stuff by the gallon when it's on sale. The drawback is you have to spend a half a minute putting the stuff in at every fill. I believe I'm getting an extra percentage or two in mpg, and possible a little more kick, but it could be my imagination. The stuff always seems to get great reviews from customers. When I run out of this batch I'm going to run the Marvelous mystery oil as an upper cylinder treatment. I've tried MM before and seems to do the same thing, it also gets very high consumer reviews. I'm probably going overboard but I really like my car and want to get her to a half million miles at least!....
Based upon the engine size and MPG ratings, each car will differ. I use Techron by Chevron. It's the only brand I use and advise others to use. It works great. If used regularly the carbon deposit will be reduced and injectors stay clean. It's all comes down to how well you maintain your vehicle. In a Prius, I could see it being a beneficial use about every 30,000 miles.
