Even though this particular query was whether you can use synthetic oil in a (older) VW, the response applies to any older car or truck that’s grown old on a diet of regular engine oil.

Why can I use synthetic oil in my older car?

Generally, the reference to synthetic oil for an engine implies the lubricant is formulated with a polyalphaolefin (PAO) base oil. PAO, which is frequently called synthesized hydrocarbon, is pure and is compatible with
mineral base oils.

These synthesized hydrocarbons, polyalphaolefins, diesters and other products that form the base stocks of high quality name brand synthetics are fully compatible with petroleum oils, which is why you can use synthetic oil in older engines. In the past, some businesses used untested ingredients that were not suitable, causing quality synthetic lubes to endure a bad rap. Fortunately, those days are long gone.

However, because the PAO base oil does not dissolve additives very well, it is generally developed with an ester co-base (usually diester and/or polyolester). The additives are soluble with the ester, and the ester is soluble with the PAO.

Likewise, the PAO tends to cause seal shrinkage, and the ester causes seal swelling, so the effects are offset when both base oils are found. It is the ester that could cause problems when you move from mineral to synthetic.

Ester base oil used along with PAO base oil in lubricant formulation is an exceptional natural detergent. In short, it will pick-up varnish on component surfaces which was deposited as a consequence of thermal and oxidative degradation of the lubricant.

Now part of the oil additives function in a synthetic engine oil, is to clean these deposits and when you move from a typical mineral-based engine oil to a typical synthetic-based oil, the varnish covering will be removed by the ester in the synthetic lubricant and become suspended.

Depending upon the build up, this suspended material could block filters, block oil flow passageways and if left long enough, lead to component starvation. The same is true for gearboxes and other industrial machines.

How do I make certain that I have no problems when I use synthetic oil in my VW?

If you’re going to change to synthetic engine oil on an engine that’s been operating for sometime with mineral oils, it’s advisable to replace the oil and filter at a reduced mileage – just to remove the pollutants that have been purged off the working surfaces.

Although this is a little off-topic; compatibility is something to take into account; whether making use of regular oil or synthetic oil. It is usually best to use the same oil for topping up as that you have been running in the engine. That is; it is preferred to not mix your oils. The reason for this is that the characteristics of additives blended for particular performance can be canceled out when oils with different additive packages are put together. For best possible performance, it is better not to have a blend of engine oil in the sump.

So to respond to the question of whether you can use synthetic oil in your older car’s engine: Most definitely, and you’ll also take advantage of all the added benefits that synthetic engine oil gives over regular engine oil.