Can Synthetic and Non-Synthetic Oil Be Mixed?

Synthetic oil is a lubricant manufactured using chemically modified petroleum components. Compared to traditional non-synthetic oils, synthetic oil provides superior mechanical and chemical properties. These include possessing very uniform hydrocarbon molecules, which gives it the superior mechanical properties it possesses, even at extremely high or extremely low temperatures.

 While the use of synthetic oil is no longer new, both motorists and operators of machinery that require motor oil still experience confusion towards its use, namely on matters regarding: 

• safely changing vehicle/equipment use from oil to synthetic; 

• safely switching back to regular oil after using synthetic; 

• mixing the two oils together. 

In the earlier days of synthetic oil mixing it with regular mineral oil wasn’t recommended because its synthetics have very different seal-well characteristics that would affect seals made for traditional oils. If the seal-swell rate of a particular synthetic was low, it could cause the seals to shrink, and the crankshaft and rocker cover seals to leak. If the rate was high, the seals tended to swell a little bit more and made the engine tight. 

 When traditional mineral oil was added to the synthetic, or used to replace it, the crank and cover seals had been worn out due to the different reactions it produced, making it unreliable and causing various oil leaks in the engine. 

Fortunately, manufacturers were able to fix this seal-well concern, and both synthetic and non-synthetic oils can be interchanged in use, as well as mixed if, there is a need to top up.