I do my own oil changes ( 1.4 TDI PD engine) using Millers recommended semi synthetic  505.01 oil and I have about 3 litres left from last year.   Millers have now discontinued this type of oil replacing it with a full synthetic oil. My car takes around 4 litres of oil so can I mix the 3 litres of semi synthetic oil left from last year with about 1 litre of the new full synthetic oil that Millers now recommend for my next oil change.

 

Thanks 

Edited 3 April, 201412 yr by alankearn

I think you'll find you'll get away with 3L putting it to the bottom of the dip stick.

 

If you want to be safe, I'd flush the engine and the fill it with fully synth.

I'm not sure if it's still the case, but some used to mix badly.

 

Since they're both millers oils, have you thought about asking them. They're usually very helpful.