Mixing Oils

[Torquemada]

Griff 500 - at the last service, it was filled up with Shell Helix 10/40. Would it cause any problems if I topped up with Mobil Motorsport 20/50 that I have leftover in the garage?

Thanks in advance.

[xTVR]

Yes, I'm interested in this one too, you know, if you get caught short and the only top-up available is different from the stuff thats recommended.
There must be an expert out there who knows something!

[WokingWedger]

xTVR said:

There must be an expert out there who knows something!

There are to many experts !

[Jaguar66]

You should check out www.bobistheoilguy.com/motor-oil-101/ It makes for some interesting reading.

Martyn

[ChimpofDarkness]

Jaguar66 said:

You should check out www.bobistheoilguy.com/motor-oil-101/ It makes for some interesting reading.

Martyn

Wow Bob can talk oil

I'll make it simple:

There are only two types of oil, mineral & synthetic.

If you add a different brand of mineral oil to the existing mineral oil in your sump, you will just get more mineral oil.

If you add a different brand of synthetic oil to the existing synthetic oil in your sump, you will just have more synthetic oil.

Semi-synthetic is just a blend of mineral oil & synthetic oil.

So yes you guessed it, you can add mineral to the synthetic in your sump and vice versa, you are just making your own semi-synthetic.

All engine oils mix, synthetics with minerals, one brand with another, & one weight/grade with another too.

The only things you may change is the ratio of additives, the weight/grade, & percentage of synthetic to mineral or vice versa.

Just make sure you keep the weight/grade and additive package right and you wont have any problems mixing any engine oils whatsoever.

Edited by ChimpofDarkness on Friday 5th October 17:55

[Bodmin]

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

Bodders

[ChimpofDarkness]

Bodmin said:

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

Bodders

Did Mr 911 buy a cheap oil by any chance?

And when did this happen, I doubt it was recently?

All quality 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 consumption

BUT 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.

[fausTVR]

ChimpofDarkness said:

stuff

That's good news, very useful, thanks.

[phillpot]

Looking at this logically, how many people (with the exception perhaps of a few enthusiasts) have a clue what oil the garage put in their car? They'll just buy a can of "whatever" at the filling station if they need a top up

[Pupp]

Just to add to the mix (sorry), some oils labelled as being fully synthetic are actually mineral based (I had it specifically confirmed by Halfords a few years ago that their 5w50 motorsport 'fully synthetic' offering was actually mineral)... seems this is ok provided the oil performs at the same level as a 'proper' synthetic. I wonder if the 'they don't mix' lore originates from the motorcycle world where two-stroke oils definitely did have issues with compatibility between castor based products, minerals and the early synthetics...

[500dread]

Pupp said:

Just to add to the mix (sorry), some oils labelled as being fully synthetic are actually mineral based (I had it specifically confirmed by Halfords a few years ago that their 5w50 motorsport 'fully synthetic' offering was actually mineral)... seems this is ok provided the oil performs at the same level as a 'proper' synthetic. I wonder if the 'they don't mix' lore originates from the motorcycle world where two-stroke oils definitely did have issues with compatibility between castor based products, minerals and the early synthetics...

Yes it's a minefield these days.

I think when this is the case the product / carton is labelled 'synthetic based' which I believe is not fully synthetic oil, which is called 'fully synthetic' but may not necessarily say so on the carton.

Be careful what you read and know what it means I guess.