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Volkswagen TDI PD oil question

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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



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".




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In my VWs I use fully synthetic oil, even the 15 year old passat.

I would not mix semi and fully synthetic oils, but many people do without much problems.

If you really care about your car I recommend you try the Quantum platinum or Quantum longlife III.

You will find that the engine runs very quietly and there's no sludge build up.

But it's about £30 for 5 litres.

In my little polo I use it every other service, alternating it with castrol or halfords fully synthetic.

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You will be good with a modern oil that meets VW spec 506.01 or 507.00. Both are good for PD engines.


It is myth that you cannot mix semi and fully synthetic. (Semi synth is a mix of synthetic and mineral anyway).


The only problem may be in sludged up engine. The sludge can hold back oil from a knackered oil seal. Fully synth is better at keeping more contamination in suspension and may remove sludge from the knackered oil seals. That is the only way you can get oil leaks from changing over to fully synth from semi.


Since you use a quality oil, do low miles and change your oil regularly, you will not have a sludged up engine.


(The above advice does not hold for most motorcycles, which use wet clutches. The improved lubricity of a fully synthetic can cause clutch slip on a bike designed for mineral or semi synthetic oil).

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