Does Synthetic Oil Cause my Engine to Leak?

One of the biggest question asked among motorcycle owners is, “Does synthetic oil cause my engine to leak?”

The question is especially asked for those who own high mileage bikes (and cars), above 200,000km. But does it really happen?

What is synthetic oil?

To recap, a synthetic oil means its base oil was made from some source other than petroleum. “Base oil” on the other hand, refers to the origin of the oil before additives are added to it. There are several sources for synthetic base oils including PAO (polyalphaolefin) and ester.

A mineral base oil originates from refined petroleum, so synthetic base oil was synthesised.

The origins of this myth

The earliest synthetic engine oils were made from ester and entered the market in the early 80s. Prior to that, engine oils were mineral based.

1. Synthetic oil cleaned out sludge left by mineral oils

Sludge is that ultra sticky brown to dark brown goo, seen in poorly maintained and/or old engines. Once sludging has set in, getting them out is no easy task, even for the engine flush.

This sludge blocks gaps and holes in oil seals, stopping the oil from leaking.

Now, synthetic oils have better flow, detergence, and anti sludging properties which went ahead and cleaned out that sludge that stopped the leak. Therefore, the engine began to leak, when it was already waiting to leak in the first place.

It was not the oil’s fault, because a diligently maintained engine, regardless of age, is not supposed to leak at all.

2. Ester

Ester has some great properties such as resistance to high heat. However, the earliest ester-based synthetic oils were said to have caused oil seals to swell and break.

Oil producers have since overcome this by using better additives to counter that side-effect. This can be seen partly in oil service standards as it progressed from the earliest SA to SN, SN+, SP, SP+ now. The API service standard corresponds to an oil’s compatibility to the latest engine tech.

3. Synthetic oils have even sized molecules

Petroleum consists of many substances and that cannot be refined out completely. Some molecules are small, some are medium, and some are large.

As such, larger molecules will clog all the niches in the engine, including places or gaskets that have actually leaked. So when synthetic oil is used, it seeps through these gaps.


So, go ahead and use synthetic engine oil. I have been using it in my motorcycle and car, which both have gone above 200,000 km will no ill effect.

Wahid's lust for motorcycles was spurred on by his late-Dad's love for his Lambretta on which he courted, married his mother, and took baby Wahid riding on it. He has since worked in the motorcycle and automotive industry for many years, before taking up riding courses and testing many, many motorcycles since becoming a motojournalist. Wahid likes to see things differently. What can you say about a guy who sees a road safety message in AC/DC's "Highway to Hell."

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