First some commonly used terms and names; SAE, Society Of Automotive Engineers. The people who set the standards, hence the phrase "make sure of the SAE rating". Over the years there have been all sorts of letters on the cans of oil, API, S, SG, SD, SDG, etc. You really do not need to be to concerned with these as all modern oils meet SAE specifications. You are much more concerned with the numbers. 20w-50, 10w-30, 5w-20. The W stands for winter and refers to the cold pourability of the oil, the final number is the true weight or viscosity of the oil. These numbers tell you the oils viscosity, which we will get into later. GL numbers such as GL4/70w-95 or GL5/85w-140 are transmission oil numbers and have no relation to engine oil numbers. A 5w/20 weight engine oil has about the same relative viscosity as a GL4/70w-90 trans oil. Remember this for later

     Synthetic oils on the other hand are man made from organic compounds or synthetic hydrocarbons such as esters or polymerized olefins. In doing this the molecules are rearranged so that they are all the same size, shape, and weight. Definitely something not found in Nature!

     This makes a synthetic product very stable and giving it optimum film strength, viscosity, and volatility resistance and allows for an integrated optimized package.

     Volatility is the ease that a liquid changes to a gaseous state. And low, high temperature, volatility is desirable in an oil. At high temperatures mineral oils rather small, compared to synthetic oils, and light molecules vaporize, shearing, much of the oil additive package goes along for the ride leaving a thick goo behind called sludge. Mineral oil does this around 350 degree f. Now remember in today's cars the operating temperature of the water is between 212 - 230 degrees f. and you have very little room for mistakes. Synthetic's oil volatility is around 600 degrees f. Add the low volatility to the great viscosity numbers and hence the much cooler running oil which means a cooler running car at any point, which means you can run more timing or lower grade fuels and either get more horsepower or better fuel mileage without doing anything else but lower internal engine temperatures.

     Another added benefit comes with friction loss. An engine is roughly divided up in a 70:30 ratio between hydrodynamic and boundary friction. Hydrodynamic means "operated by the force of liquids", your oil. An internal engine relies on the fact that there is oil between all moving surfaces. This is supported by hydrodynamic pressure. Synthetic oils reduce the hydrodynamic friction caused by the hydrodynamic pressure because of its molecular uniformity and heat stability. This means more energy at the wheels as we lose less in the engine trying to push against the higher hydrodynamic pressure of mineral based oils. Less work means lower temperatures, which means more horsepower at lower temperatures which means I can start the whole cycle again of trying to get better mileage or more horsepower.

     One more thing that synthetic oils does that mineral oil does not is that it's molecules are attracted to hot high-pressure surfaces where its smaller molecule chain, not smaller molecules, and high-quality anti-wear additives can actually infuse themselves into the metals pores. You have all seen the add where they take a car fill it with synthetic and run it around the track. Then they bring it in drain the oil and send it back out...Well this is the reason it works. The friction loss gain using synthetic oil instead of mineral oil is about 4%.

     A quick look at synthetic gear oil, some conclusions and we are done.

     Mineral oil has a higher viscosity as we have seen, this causes higher energy loss in gear boxes too, also the higher film strength of synthetic oil is very important in gear boxes. Also synthetic oils shed from gears quicker than mineral based oils thus cooling the gears better insuring longer gear and bearing life. This also means that oil does not puddle in gear teeth lowering the chance of binding. The energy reduction gains in gear boxes and diffs is something like this... 0.5% for each gear reduction stage and 1.5% for hypoid gears {diffs}

     So what does this mean to you and should you use synthetic oils? First the net gains are improved performance and fuel mileage. Will you notice a giant difference? No, but your engine or gear box will. You will be using a product designed just for automotive use, not something that just happens to work well. I doubt of any of you are still using what passed for axle grease in the past century, why stay with an oil as old as that?

     But it causes oil leaks! No it does not. Think on what you just said. How could an oil cause a mechanical oil leak? What you have is an oil that is the same viscosity through a large temperature range, not like what you have in there now. So it flows quickly now! Not like the thick stuff you have in there which would not flow through a hole no matter how big at the cool temperatures. It does once it thins at high temps. Thus the wet engine with no real leak! I switched to synthetics and started burning oil. Same thing. You were already burning oil as it got past worn seals and rings but it needed to be so hot to be thin enough to flow you did not see it smoke as you were flying down the road. Guy breathing your exhaust may be able to tell you different though. My oil pressure went down. Your right at cold start up it did but at full operating temperatures it is the same or in extreme heat higher.

     It will mess up my overdrive and attack the brass in my trans. I have heard this for years though in all the years that I have been using it I never found evidence of it so I talked to the engineers at Amsoil, the oldest producers of synthetic oils, and Castrol. In very plain English POPPYCOCK! The oil and additives is totally compatible and if you have problems you already were on the way there and cleaning all the mung and sludge out brought it to the surface.

     So, use it. I use it in my MGA twin cam, my father has it in the XK120. We have it in customers SS100, XKE, XJS, Austin-Healey, MGB, MGT, Bugyes, Volvos, Jeeps, Fords and all sorts of things!