I currently have Semi Synth oil in my MR2 and it is about half full at the mo. I have a couple of litres of Castrol Fully Synth spare here and just wondering whether to top it up or not.

It isn't essential as the next service is only 3k miles away, but wondered if you can mix synth/semi-synth or not?

DW

 

I don't think it's a very good idea - they might not agree with each other . Semi & semi is a much better idea!

 

I have mixed semi synth and fully synth before, and haven't had any problems yet. Surely semi synth is just a mix of normal (mineral?) oil and synthetic oil, so adding fully synth just ups the concentration of the synthetic oils.

Might be completly wrong though.

Chris

 

AFAIK The only oil you shouldnt mix is MOBIL 1 and other makes, not sure why but i was taught this when i did my apprenteship...
Rob
Was never warned about others....

 

There should be no problem, after all, people pour all sorts of additives and stuff into the oil- sometimes (but not often) to good effect. Strangely, it's not recommended that gear oils be mixed.
Back in the "good old days" (?) mixing normal mineral oils with castor based oils (eg Castrol R ) was a no no.
JohnD

 

There will be absolutely no problems mixing semi synthetic oils with fully syn and even 'normal' mineral based engine oils. By FAR the most important aspect is that there is sufficient oil in the engine at all times, irrespective of the quality. I would be more worried about knowingly running your engine with a low oil level for 'only 3k miles' - top up today if you haven't done so already. I've got some lawnmower oil you can have if you don't want to 'waste' the fully synth stuff...

 

Toby, the oil is not low - it is well above the half way mark, and considering it was full over 3k miles ago, I am sure it will last another 3k easily before even reaching near the bottom mark.

I dont like to overfill cars with oil for weight, overfilling etc etc. And my car never uses a drop of oil anyway really (MR2 by the way)

Nice surname (same as mine)

DW

 

Check the funny small print letters on the two bottles. This should give you a clue as to what will mix with what. Match up the letters and they should mix - At least that's my understanding of it...

Personally, I wouldn't mix oils.