Is it true that once you've used fully synthetic oil you can't go back to semi-synthetic or mineral oil without flushing the engine?

Or is it a load of B@ll@x

B@ll@x

Deleted by MODVRS

Edited 20 June, 200917 yr by MODVRS

The only thing that comes close to that is that on an engine that's been run for many thousands of miles on mineral oil, sometimes a change to synthetic will cause oil leaks, as the synthetic oil dissolves the "varnish" built up by the mineral oil, and starts to work its way out through seals.

As above there is no issue moving from one to another and back again.

Cheers

Guy.

Old wives tale.....my dad's a former mechanic and he tried telling me this.

Our argument went on for several minutes.

Once you've had... oh, hang on, that's something different!

Don't Ester oils do something funny when mixed with mineral oils though?

Nope, there fine too. The only downside is ester oils tend to be high quality, mixing it with a mineral oil will dilute this.

The one you cant mix is castor based oils.

Cheers

Guy.

Thanks everyone....Now what about ky gel and baby oil?..........

KY,Baby Oil,Esther........Oooh Err

Thanks everyone....Now what about ky gel and baby oil?..........

Baby oil will melt a nodder, KY wont ........ hth

Baby oil will melt a nodder, KY wont ........ hth

Yeah, thats one mistake I won't make again...

Baby oil will melt a nodder, KY wont ........ hth

Dont I know it, :(

Check the avatar

Don't Ester oils do something funny when mixed with mineral oils though?

Yep potentially they can if the mix is high enough.

The 'theory' is that generally additives in mineral oils have a di-polar nature. The mineral oil itself is non-polar so the polar end of the additives prefers the [polar] metal surfaces of the engine rather than the oil so they 'adhere' to the metal surfaces. The non-polar part of the additive prefers the mineral oil to the metal and 'remains' in the oil. The additive effectively bridges from metal to oil and helps the oil adhere to the metal.

Ester oils are polar in nature and also like to 'adhere' to the metal surfaces rather than the mineral oil. Since the ester oil is present in a larger quantity than the additive in the mineral oil it takes up the space available to the additives and prevents the additives from finding the metal surfaces. The end result is that the lubricating effectiveness of the mineral oil is lower.

Mineral oils also contain VI [viscosity index] modifiers that are longer/ controlled chained polymer molecules used to control the viscosity of the mineral oil. The problem is that the ester oil can disrupt these molecules and affect their behaviour. Rather these polymer chains being long 'snake-like' orientated they can pucker up into ball shapes and screw up the viscosity / flow behaviour of the mineral oil.