87'GtStang

I honestly wouldn't bother with the "blends" out there. When you think two formulations, mix them together, then say they are blended, you probably think 50/50 mix - wrong. The typical oil industry usually blends at best, a 30% concentration of synthetic oil with conventional. It just isn't worth the money for how much more they mark it up (best profit margins for the oil companies). If someone gives it to you, came in the car, or you got it on a good discount, go ahead. If you cannot afford full-synthetic, but want to run at least a "blend", you are better off doing something like buying equivalent petroleum and synthetic blends and mixing them yourself; ie: mobil 5000 and mobil 1 of the same viscosity. You will come out ahead in the long run versus buying that so called blend with only up to 30% synthetic oil in the bottle.

Nuke

Absolutely not. By blending on your own, you have no idea how the additive packages in each oil will interact with one another. Synthetic blends sold by the leading oil companies formulate them as a single entity for a reason. Each has their own variation in additives but it's these additives that provide you your protection; not just the base stock. Feel free to mix if you want to be just be aware that you're taking a risk, regardless if both products are made by the same company.

87'GtStang

Engine oil's purpose: clean, lubricate, transfer heat, resist rust, prevent formation of acids and sludge, and a few others. All engine oils are on the same mission, weather it be castrol or super-tech. I believe one or the other should be used, petroleum or synthetic. Mixing is common procedure in some applications however, mainly for cost. Different oils do have different formulations, a manufacturer however will throw a lot of money at a certain additive and place it in almost everything they sell. Throughout different product lines, different concentrations of this additive are placed in them, the higher the concentration, the higher the price.

By saying "Absolutely not" to blending/mixing, you basically are shutting down and condoning the entire aftermarket oil additive industry, as they would have no purpose if this were entirely true. You also better hope that never are you on a road trip where you check your oil level, are 3 quarts low, and the quickie-mart doesn't have your oil brand - as this would be considered mixing!

Do I mix my own oil? No, I have no reason to as I get good discounts on many oils, some even free. Would I though? If I had to have something very high quality like a PAO, polyester, or diester additive in my oil but couldn't afford the stuff because they are about $9 - $12 a quart, then yes I would mix. I am however a person of in or out philosophy, I don't see a purpose in going only halfway.

As stated, "mix at your own risk", but it won't be hurting anything......