Monday, January 23, 2012

Better Mileage Through Synthetics

I have wanted to post about this for a while but haven't gotten around to doing so. As you I am a big believer in using synthetic fluids in vehicles. One of the benefits that I had mentioned was improved gas mileage. As evidence I offer up my 96 Jeep Cherokee with 370,XXX miles on it. 

When I acquired the vehicle last summer it has standard fluids in it. The dealership I purchased it from had changed the oil but had put in non synthetic oil. I had done some tune up work on it like changing out plugs, plug wires, distributor cap, rotor, air filter, coolant, and fuel filter but hand't gotten around to changing out any of the fluids before going pheasant hunting. When I left I topped off the tank and filled it up again after going 186 miles and put in 9.3 gallons. On the way back I put 185 miles on it and put in again 9.3 gallons. Shortly there after I manged to get around to changing the gear oil in the transmission and differentials as well as fluid in the transfer case. The engine oil is still the non synthetic fill from the dealership. This time when going deer hunting in colder weather I drove 187 and 183 miles between fill-ups and this time only put in 8.3 gallons each time. 

Those 2 gallons of gas alone come close to covering the difference in the cost between standard and synthetic fluids that I put in. With the other driving that has been done the cost of those fluids have now been easily recouped. Granted this is just anecdotal evidence but does help to support the claim of better mileage by using synthetic fluids.

When it is time to change the engine oil I should see more of an increase in mileage but I don't know how much.  Now before I change the engine oil to synthetic I will be changing the oil pan gasket and valve cover gasket as they are old and are a bit on the leaky side so that should eliminate any of the associated problems people claim are caused by synthetics.


  1. What about semi-synthetic oils? Mechanics pour them to my 16 yo car with a gasoline engine. Should I try a fully synthetic oil anyway?

  2. I would say give it a try. Semi-synthetic oils are just that a blend of standard oil and synthetic. They offer better performance than standard oils but are cheaper than full synthetics. The down side is that a semi-synthetic is more expensive than a standard oil and don't offer as good of protection as full synthetics. Given the age of your vehicle I would suggest going with a high mileage full synthetic as they would minimize the oil leakage problems that may happen. The high mileage oils have additives that that cause the gaskets and seals to swell and stop some leaks. If your vehicle is currently leaking oil from the valve cover gasket or oil pan I would suggest getting those replaced before switching to a full synthetic as full synthetics will leak more. When you do switch be mindful of oil consumption as synthetics may clean out deposits that are currently plugging leaks. So for the first couple of oil changes pop the hood and check the oil every time you get gas and carry around an extra quart in your trunk in case you need to top off the oil. As far as putting in other full synthetic fluids I would highly recommend it. The one place of concern is if you have a manual transmission as putting in synthetics may make shifting more difficult. The reason for this is the synchros are dependent on there being a certain amount of friction to match shaft speeds. A synthetic fluid with its lower friction and better flow properties may impact the synchros ability to do this. I haven't this problem with the vehicles I have done this to (my wife's Volkswagen, my Jeep, my previous Bronco II, my previous BMW) but I have heard some people say they have had problems after doing this. If this happens switch back to standard gear oil or try a heavier gear oil like 75w140 instead of 75w90.