I have a feeling the answer to this question is "NO" based on the very extensive threads on "oil blending" I just read but I was hoping to confirm.
Would it be all right to use a few quarts of one full synthetic from a manufacturer and one quart of full synthetic from another manufacturer if the viscosities are identical?
The "Oil Blending" threads appear to be dealing with mixing viscosities which I don't plan to do.
I'm guessing the answer is "NO", however, given that different detergents and base ingredients may vary from manufacturer to manufacturer and therefore, similar oils shouldn't be mixed.
Can anyone opine?
(Oils in question are: Spectro Platinum 4 10w-40 and Castrol Power RS Racing 4T 10w-40, both full synthetic).
You are on the right track and there are a few facts to consider.
In simple terms.
1, One of the main things regarding API certification is that it is conditional for an oil to gain certification that it is "miscible with other oils". That only means it can be simply mixed readily with other oils that are also API certified.
It works out great for a top up on the run between services if the oil level is down too much and probably the only major benefit of API certification.
It doesn't make, nor can it make any guarantee about outright compatibility or performance in service.
2, In recent times, oil formulations have become quite complex in terms of diversity and complexity of base oils and additive packs and blends.
Although there is a fair bit of similarity in general terms, there can be no guarantees nor can anyone make an outright guarantee. It's impossible certainly in this day and age.
Gone are the days of only one or two major type of basestock with simple additive packs that share quite a lot of commonality.
API certification does help to provide some confidence in the miscibility department.
But we now have 5 distinct groups of oil basestocks with about 7or 8 different types of basestocks which can be totally different in terms of what's required to make a "fully formulated oil".
3, Whilst it's possible for a right thinking person to assume it's ok to even mix different viscosities to achieve a desired outcome with a franken brew if one wishes to play Tribologist at home with your ICE chemical reactor.
There are no guarantees as to the outcome of the resultant formulation.
As far as I know, there have been no major engine failures directly attributed to a bit of mixing of oils.
Certainly the performance of fully formulated oils varies greatly between them, and it is testament to the quality of the basestocks, additive packs and the ability of an oil company to blend a finished oil to a performance standard.
Blending forms a core aspect of the true art of finished oil production.
If you don't care much about the machine other than to keep it going well then mix away.
Otherwise for the very best results and confidence, try to stick with one grade and or type of finished oil within a given range of your chosen/favourite brand.