I was always told that you should not mix mineral oil with a fully synthetic as some types of rubber react & swell
This was the result I saw when a Porsche 911 owner added a mineral as a top up when he couldn't find any fully synthetic.....oil started seeping from several areas of his engine.....expensive mistake
Did Mr 911 buy a cheap oil by any chance?
And when did this happen, I doubt it was recently?
modern oils (synthetic & mineral) are fully compatible with the elastomeric materials that engine seals are made from, and you shouldn't have any issues with leaks.
Here's the current thinking on the subject of mixing mineral and synthetic oils. This information is based on the answer to a technical question posed on the Shell Oil website:
There is no scientific data to support the idea that mixing mineral and synthetic oils will damage your engine. When switching from a mineral oil to a synthetic, or vice versa, you will potentially leave a small amount of residual oil in the engine.
That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other. (The exception is pure synthetics. Polyglycols don't mix with normal mineral oils.)
There is also no problem with switching back and forth between synthetic and mineral based oils.
In fact, people who are "in the know" and who operate engines in areas where temperature fluctuations can be especially extreme, switch from mineral oil to synthetic oil for the colder months. They then switch back to mineral oil during the warmer months.There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if you had used one product or the other for a long period of time. People experienced problems with seals leaking and high oil consumptionBUT CHANGES IN MODERN ADDITIVE CHEMISTRY HAVE TAKEN CARE OF THOSE ISSUES
This is exactly what my oil chemist mates at the Castrol research facility told me, if there was any risk to mixing oils make no mistake they would be telling me.
Pretty conclusive I'd say.