With the refrigeration and A/C systems, we had to replace the mineral or alkylbenzene [MO or AB] to poly ol ester [spelling?] [POE] when we retrofitted from CFCs or HCFCs. mineral has an affinity for those refrigerants and has good miscibility which facilitates oil return. There are a considerable variety of compatibility considerations between oils, refrigerants and materials of construction that came along with retrofitting existing systems. In that area, MO and AB seem to be okay in some proportions but where POE oil / HFC refrigerant combinations require nearly no [or at least less than 5%] residual MO. A variety of problems otherwise including lubrication problems and have observed evidence of that.
For cars I've always used the base [?] oil, cheap on sale usually from Canadian Tire branded Penzoil, Castrol or Quaker State or Canadian Tire brand [watched Esso Strathcona go from filling Esso brand to Motomaster brand, same splursher].
I bought a '77 Impala with a 350 / 4 BBL Rochester, carburated anyway, where I believe the seller when she told me that she following her brother's advise - because he sold cars... alright, I know now that I write it down I sound really gullible - she only used Mobil 1 and so I could only use that oil going forward.
I said thanks, okay, changed it over to splursh, ran it a bit to get it to [not] mix, and did a second MO change. The engine had over 100,000 miles on it and was extremely powerful [comparatively, had all the big blocks ect] and ran exceptionally smooth. It didn't go through any appreciable amount between changes and I believe it had something to do with maintenance and the synthetic oil.
What kind of concerns me is the semi-sythetic blends [or whatever they're called], I just can't imagine this being a good idea, and what happens if you get a mix of base and semi-synthetic, and there's been times when they're out of stock on the base types [sale or no sale] and all that's available is these types.
There's no getting around that product positioning is getting you to notice the abundance and pop of these semi and full synthetics while your cheap old grandpa oil only occupies a few bottom corner rows now.
Reading the labels probably says - warranty approved - buy me - pay more - I'm better, but I'm sure some of the members here must have raised an eyebrow when these blends became popular. Goes for Cars and Bikes. Wondering what others think, why all of a sudden [or maybe closer to everything else I've been ignoring - the last couple decades] is it okay to mix base with synthetics, and brag about it?