Since I bought my 1.4 TDI PD VW Polo in 2002 (yes I know its old but its only done 17,000 miles) I have changed the engine oil every year using the recommended 5w40 semi synthetic 505.01 oil (Millers). I have always bought a 5 litre container of oil but the engine only needs about 4 litres so I always have about a litre left, this builds up so about every fourth year it has built up so I dont have buy for that particular year.I now have a build up of about 3 litres and as it is near to oil change time I went to buy a 5 litre container of the recommended oil only to find Millers have discontinued it and say I should now use their recommended full synthetic (not semi) oil

See here

Millers Oils XFE - PD 5w-40 Semi-synthetic Engine Oil

So now to my questions

I presume it is OK to mix synthetic and semi synthetic oils.

If the answer to this question is no would it be OK to mix a different manufacturers semi synthetic oil with the Millers oil

ALSO

I am sure a few years ago I read that if you had used semi synthetic oil for a number of years and you changed to full synthetic there was a chance you could get oil leaks, any truth in this or is it a "old wifes tale".

Thanks