3.8 - Synthetic Lubricants ( Mark Stephenson,  )

I have used synthetics for many years, and I have found (one) thing. If an engine has a small leak, it will leak more with synthetics. Here is my understanding: Petroleum products are made up of hydrocarbon chains. The thicker the oil the longer the chains; the thinner, including volatile petroleum products, such as fuels, are made of short chains. In crude oil, there is no uniformity of the chains. Refining crude works kind of like sorting rocks through screens, you get a a general uniformity, but you can still have some intermixing of the chain lengths. Synthetic oil is like making the rocks from scratch, so every chain is exactly the same.

The varying chain lengths in refined oils are what causes refined oil to vaporize quicker at high temperatures and thicken more at lower temperatures.

The varying lengths also make the oil less slippery. In fact, synthetic oil makers used to (I don't know if they still do.) advise against using it in new engines, because the rings and things would take much longer to seat, i.e. the engine wouldn't break in. Years ago, in a lesser vehicle, I had a long grade I used to climb on a regular trip that I took. I would crest the hill at barely 50 mph with refined engine oil. When I replaced the engine oil with synthetic, I would reach the top at 53-54 mph. When the transmission was due for a change at about 36,000 miles, I used synthetic gear oil (whatever was recommended), and I crested the hill at 56-57 mph. This is an average over many trips.

So that was proof to me of the reduced friction of synthetics. In addition, I have not had an internal engine failure on any vehicle I've used synthetics in, although that is not a statistically significant sample.

As to the leaking question, I can only surmise that the reduced friction makes the oil flow more easily, and that would include flowing out tiny cracks and crevices. At the same time, the shorter chains in refined oils will vaporize over time, making the oil thicker and less likely to leak.