I've looked on here already for answers but to no avail. I have a 67 MK2 340 into which I normally pour a brand of mineral oil from Halfords. I also have a collection of other oils synthetic/part synthetic from other cars - is it safe or advisable to use these to top up my car ? can these be mixed with mineral oil ?

 

Synthetic and conventional oils can be mixed. Naturally, you're diluting the benefits (real or perceived) of the synthetic...but nothing bad will happen.

Cheers
DD

 

Try to stick to the viscosities as outlined in the User Manual, (if you have one), otherwise stick to a traditional oil like 20-50, or 15W40.

I would not put in the new oils like 5W30 or similar as used in my X358, the working clearances on the XK were larger than on modern cars. Indeed, the XJK engines were all made on secondhand machinery and even specified clearances were not always maintained.

The effect if you do use modern "thin" oils is to likely lower the oil pressure. Of course with modern oils you will probably get away with this, but, of course, may not, and may end up with failed main or big-end bearings. I would not take the risk if it was me.

 

Synthetic oils are not always "thin", you can get 20W50 fully synthetic.

We will end up opening the debate about additives etc in a minute , I agree that synthetic and mineral can be mixed, and also that you should keep with the same or similar viscosity, but try and avoid high detergent oils if you have the old rope seals, as they can wash the seals and make them leak more.

Oh no have I started the oil debate again !

You will find a lot of discussion re oils on the forum.