I believe he is saying as long as they have the same certifications, ratings, etc.
With regards to API starburst, yes.
Sorry @erhayes, I chose poor wording in my original post.
It's not there's a physical test, but more so in order for an oil to achieve the API starburst, it must be miscible with other API certified lubricants. The basic chemistry between various lubes is very similar between Company A and Company B, while the additives and base group(s) may differ, the physical chemistry of each completed product is similar due to the miscibility factor that the API imposes.
You can (and many do) also mix various weights. For instance, I have 3 gallons of 10w30 sitting on the shelf. In the summer, when I run through my 5w30 stash I will be using two quarts of 10w30 and three quarts of 0w20. This should result in a viscosity that is comparable to a thin Xw30 or thick Xw20 depending on the specific cSt values of each oil.