Mixing fully synthetic and semi synthetic oil

[SurreyGuy]

I put fully synthetic oil in my girlfriend's car. She ran low on oil yesterday and (bless her for thinking for herself) topped up using semi synthetic oil, albeit of the same viscosity.

Will this do any harm to her car?

[Panic Button]

Well, since semi is already a mix, I don't see that it can be a problem

[Nkakamasembe]

Non at all. Some school of though have it that semi synthetic oil is better than fully synthetic.
If your motor is still under warranty and for some reason parked up your oil sample test could just give someone the one reason to reject a claim.

[logic]

Should be OK both good answers above,
Only experience I have had is when I once mixed mineral with fully synthetic, It turned into a snotty substance,

[SurreyGuy]

Thanks folks - I might not tell her it's OK for a while, though! :o)

Nkakamasembe - the car is nearly 15 years old, so any warranty is long expired!

[Nkakamasembe]

The trouble is when you give an all clear than something unrelated to the oil top up goes wrong! the oil top up becomes the first suspect. OLDER MOTORS RUN ON HIGHER TOLERANCES ANYWAY so need not worry ON THIS TOP UP.

[Rickyjohnston]

No point in putting fully synthetic, which is expensive, in a 15 year old car. You'd be as well running on the cheapest 10/40 mineral oil you can get your hands on.