STP Oil Additive good or bad?


Just out of curiosity, what do you guys think about those Oil additives that STP and others make ? I was wondering if it would benefit my Elderly Honda engine or if it would do more harm than good. Thoughts and ideas? How about the “complete” fuel system cleaner ?

Steel building for an car garage!

STP is a “Viscosity Index Improver”. Translation: It will make your oil thicker.

How thick? Nobody can predict this, as we don’t know the current viscosity of your oil. All I can say is that I would personally avoid this stuff, as you could wind up with oil that is too thick to flow properly, particularly if temperatures drop. And, if oil doesn’t flow properly, you could wind up with lubrication problems, and that could spell an early demise for your engine.

As to a fuel system cleaner, if your engine is in need of this, the only two brands that I would recommend are Seafoam and Techron (by Chevron). The other fuel system cleaners don’t do much of anything except lighten your wallet, IMHO.


I always recommend against STP, unless you have a wornout engine and you want to get one more summer out of it. I’ve never used STP since my student days car (57 Plymouth, well worn) and have not overhauled an engine since the summer of 1964. Modern oils have all the aditives you need. Fuel system cleaners (the ones that work)are a good idea every 30-40,000 miles or so.


I have a slightly different take on STP. Many ago I was responsible for all operational decisions and work in a very large high speed production plant. I had my guys just finish a major “to the floor” overhaul of a production line. We had just started it up and were running it without making product and at low speed to work in all the hand-scraped main bearings etc., etc. When I told them to speed it up to full speed I had them run it for and hour or so and on one of the machines in the line driven by a 50 HP motor with a forced oil lubrication system I had our electrician check the current draw on the main drive motor. I then had STP put into the crankcase according to the mixing instructions and after it was thoroughly mixed into the motor oil I had the electrician check the amperage the motor was drawing. I do not remember the exact figures but the current draw was about 20% less. Since only about 10 minutes had elapsed between the two and since the speed was not changed I concluded that the significant reduction in amperage was the result of reduced friction. This machine had three large crankshafts that operated the mutitude of mechanisims on the machine. To me reduced friction was good - very good!
So based on this I believe that dismissing STP as worthless is open to discussion.


A production floor INSIDE is far from real world experience. Tell me how well STP works when you try to start you car at 5am after it’s been sitting all night at -15. IF it even turns over there’s a very good chance it’ll be toast long before the engine reaches operating temps.


You don’t need this additive unless you have a near worn out engine.
Mike is exactly right about STP and cold winter days. It’s going to work the battery and the starter motor much harder; if it starts at all.
Many years ago I worked for a small town Subaru dealer. Subaru at one time required, and paid for, an manual transmission oil change at the 1000 miles mark along with several other little chores.
Our sorely lacking service manager INSISTED that every trans oil change (they used 80/90W hypoid oil) also get a can of STP oil treatment with it.

You can just imagine what shifting a transmission was like on a zero degree day, much less a 15 below one, with STP diluted 90 Weight gear oil.
The best analogy would be rowing a boat through a tar pit.
One could even notice it when the car was decelerating since it gave the feeling that the brakes were half-applied.


For the oil thickening, you don’t have to mix anything, pour anything into a running engine or jump through any hoops. A can of straight 30W motor oil is the same price as any other quart of oil. Two quarts will thicken the mix even more. Don’t work harder or expensiver; work smarter. If you have the CVCC engine; mine did well on five quarts of 30W and got 36 MPG highway. 1.8 engine in an 82 Accordion.


Two facts. A friend of mine had a '78 Chevy van with a 350. He changed the oil twice a year and always used STP. last time I saw that van it had 380,000 miles and was still going. It was on it’s third transmission… The heads and oil pan have NOT been off this engine.

When resizing cartridge cases in a die made for that purpose, Only two lubricants will work to keep the brass cases from sticking/jamming in the die. A special and expensive die lubricant or straight STP. Other oils, even 90 weight gear oil, will not prevent sticking like STP does…

STP. Scientifically Treated Petroleum… So what is the downside? When it burns in a worn engine, it leaves a LOT of deposits on the spark plugs and exhaust valve stems…


What about your elderly Honda engine do you believe needs help?

If it is not broke, don’t fix it.


What year honda? What size? How many miles?


STPs label once read THE RACER’S EDGE. Well, racing engines are LOOSE engines. The clearences for pistons and bearings are very loose and require a great volume of oil and a thicker oil. Late model engines are built to very close tolerance with very tight clearences which call for thinner oils. I have seen Japanese engines fail to run when temperatures dropped below 10*F due to thick oil pumping up the lifters, preventing the valves from fully closing. Also,when tearing down a seized 4 cylinder Ranger engine one cold January I found the oil pump drive shaft twisted into a cork screw and lying in the pan. The driver used 20-50 oil and one of the molasses additives.