Can I add a little 0W40 to 5W40?

[JavierH19]

Quoting myself from another thread with a few tweaks here:

Mixing oils isn't "optimal" because the odds of you improving something are infinitesimally low but negatively impacting some aspect of the chemistry, quite probable. It could be a reduction in low temperature performance (most common), an increase in Noack, throw off the FM balance...etc. Lots of things to "go wrong" which is why oils are fully formulated products and not Jebediah with a butter churn and some raw ingredients tossing this stuff together in a shed.

That said, unless you actually put the mix, and the products you mixed individually through the testing sequences, you really have no idea what has changed from the performance of each of those constituents alone. It's not something Joe Average is going to be able to determine from sight/smell/taste/feel or from a UOA and the odds of it having any easily discerned impact on longevity are low. Rings coking up, varnish build-up...etc, these things don't happen overnight or even over the course of a couple (normal) OCI's.

In the case of being frugal and using up surplus product, while I generally don't recommend mixing as a regular practice, what the OP is considering doing is quite reasonable.

Well yeah. It's the reason why I said "doesn't mean that the worse of the two won't lower the effectiveness of the better one" I've mixed 15w-40 with other 15W-40’s and euro oils with other euro's and I will do so again and not worry. The spread between them is minimal whereas the spread of mixing 0w-8 and 20w-50 is obviously massive.

 

[wpod]

Pour the oil in the engine with your left hand.

 

[Triple_Se7en]

Mix away.... won't hurt anything during a typical 5k OCI.

 

[Sonic]

I mix oil all the time. Last change on one of my vehicles I used 20w50 motorcycle oil mixed with 0w20. I don’t do it because I think I’m making something better, but because I have so many partial oil jugs open that need to be used. I made a 30 grade oil.

 

[SR5]

I did 6k miles on a FrankenBrew of Penrite Vantage 10W40 semi-syn, Valvoline SynPower 0W40 full syn, Castrol Magnatec 10W30 semi-syn, Castrol Edge 10W30 full syn and the last of the Valvoline DuraBlend 10W40 semi-syn.