TL;DR: Added Marvel Mystery Oil 200 miles before oil change. It seems to have cleaned out some sludge and has improved oil flow to the top end. My mileage improved 7% (before 13.5 to now 14.5 MPG).

• I bought a 1995 F150 4x2 4.9L / M5OD with 292,000 miles. The PO put in a "fail-safe" thermostat that failed open -- as it was "designed" to do. I replaced it with a 195*F Stant thermostat and the truck ran much better.

• The truck was initially getting 9.5 MPG. I've done routine maintenance and replaced all vacuum lines. Mileage improved to 13.5 MPG.

• Side note: I hate "fail-safe" thermostats. Every time I've bought one, it's failed within 100 miles.

• Because the old thermostat never let the truck warm up -- for who knows how long -- the engine is full of sludge. For example -- I can see two rockers through the oil fill opening. I would see some oil on the rockers, but not much.

• I added 16oz of Marvel Mystery Oil (MMO) to my oil a week ago. The oil I'm running is Castrol GTX high mileage 10W40 with 16oz of Lucas oil stabilizer.

• Since adding the MMO, mileage improved to 14.5 MPG! I'm not saying MMO is responsible for the improvement -- it is just "snake oil" after all.

• But it's pretty impressive that mileage bumped up to 14.5 -- that's a 7% improvement! The engine is also quieter and (honestly) it seems to have smoothed out.

• I looked through the oil fill opening last night and, to my surprise, I see plenty of oil on the (visible) rockers now. As in significantly more than before adding the MMO. Presumably it's been cleaning out the lifters & pushrods, allowing more oil to make it to the top end.

• I will be changing the oil this weekend. I plan to use Rotella 15W40 this time, and I will not be adding MMO initially. I will probably continue to add it a couple hundred miles before an oil change though to continue cleaning sludge out.

• While the MMO worked for me, it probably won't work for everyone. There is "risk" with using it -- sludge can migrate to places it shouldn't be and cause big problems. It thins the oil out, potentially to the detriment of lubrication quality.

• For a $5 bottle of "snake oil", I am really surprised with the improvement to top end oil flow. The jury is still out on whether the mileage improvement is due to the MMO cleaning the engine out or maybe me being more careful while driving because I'm nervous about it thinning the oil out.

 

I have used Marvel Mystery Oil for decades....still do. Snake oil or not it keeps my rebuilt engines clean as can be. I have grown to trust it.

 

Good for preventing stuck lifters, sticky rings. Good for older iron that isn't driven much. It's a light machine oil with a dose of penetrating oil (wintergreen) Not saying anyone should use it or not use it, but it seems to work best when used in both the fuel and oil. 4oz. per 10 gallons. It's cheaper by the gallon.

 

I use MMO once or twice a year in my oil and both gas tanks. (usually a few hundred miles before my next oil change) That stuff really does work well.

 

The only place I use MMO at the moment is as an air tool lubricant. Good for Ingersol-Rand impact guns.

 

Hi,
I used it when I ran "Dino" oil.

I don't use it with Synthetic Oil.

It is a cleaner, as you saw!!!

 

Never ran it in the oil, but have used it in fuel............also have dumped some down the carb while the engine is running to de carbonize valves, piston tops and combustion chambers.........it works for that and keeps pesky mosquitos at bay while you are doing it........smokes like hell but works!

 

I've also used it in fuel tanks. Like my lawn tractor, generator, and even my Harley. Things that tend to sit more then run.

I bought a vintage 70's Onan generator. I started putting large dosages of MMO in the tank. After a summer of usage I pulled the carb apart. It was spotlessly shiny aluminum inside. I was impressed.

 

Thanks, everyone, for the responses. As a 30 year old novice, I'm glad to see that my "discoveries" are not new. Don't let my old thread fool you, I have no idea what I'm doing (as if you couldn't tell).

I'm amazed at what the MMO pulled out. Just finished up the oil change. Probably 300 miles of running a bit of MMO. Added 16oz more and let it idle for about 20 minutes, then changed oil while it was hot.

The oil was black. And the oil filter was "heavy" even after it drained. I saved the filter and will cut it open tomorrow to see what's in there. Seems like the MMO dissolved a decent amount of gunk. I'll post some pictures of the oil filter innards tomorrow.

I ended up using Mobil 1 15w50. In case anyone is interested, Wally world has 5 qts for 24 bucks. The label says it has more zinc than normal oil which should be a good thing for the cam+lifters. It's a full synthetic, and I'm shocked at how cheap it was.

Anyway thanks again for the responses. I'll post some post mortem pics of the oil filter tomorrow.

 

 

I used it in my oil once and it worked well. I only ran the engine for maybe a couple hours and the oil came out pretty dark. ​​​​I run seafoam in the gas tank every once in a while too.

 

I've been using it as long as you have been alive,my Granpa and Dad got me to using it. Helped both of them growing up and they are/were both wizards on engines and such. Any vehicle i buy that is used gets the engine oil system cleaned out with it and I run it in the tanks. Rear tank on my 93 is dosed with it now as I haven't been driving the truck a lot and have just been using the front tank.It works well as a fuel stabilizer too, It's not as harsh as some other cleaners and it does it's job well.

 

My uncle got me into MMO a few years ago. I run it with the fuel every tank mainly because of ethanol. I don't notice it so much with my engines but in dad's truck he gets 1-2mpg better with MMO then with out.

 

wonder if the better mpg is due to it thinning out the oil

 

I wouldn't think so. It has more to do with removing the carbon and making a better piston seal,increasing cylinder compression when used through the fuel system. Used correctly in the oil you put it in a few hundred miles before a oil change, I have never been one to replace a quart and run it the whole interval.
I have used Rislone and even kerosene before to flush an oil system on a car. Thats usually a dump and pray situation on a beater.

I normally use SeaFoam to clean a fuel system out and then MMO in the tank every so often afterwards to help keep it clean. I don't use it to try to increase MPG, just as a preventative cleaner,any MPG increase is secondary.