Blended Oil Cocktail, simple Q for smarter people
 

Ok, so I've started doing my own oil changes. My preference, after much review of the info here and elsewhere, is for Castrol Syntec 5w40, which is rare as hen's teeth around here.
Mobil 1 in 0w40 (Porsche approved) and 10w40high mileage is readily available and relatively cheap. If I blend the M1 0w40 and 10w40hm will this result in an appropriate 5w40 weight oil for my '01S?
Or suck it up, pay the extra, and order the Castrol in the correct weight?
Thanks in advance for the education.

Maybe. Different oil manufacturer's often start with completely different blend stock bases, then use their own additive packages to put the oil into the weight characteristics they want. So blending two different brands of oil could easily take you just about anywhere viscosity wise.

You would be vastly better off not playing "mad chemist" and buying the correct oil. Either go for the Castrol 5W-40, or better yet the Joe Gibbs DT 40.

+1 based on memories of a fuels and lubes class I had about 30 years ago.

Synthetic oils start with the lower viscosity base stock with an additive package that brings the higher viscosity when hot, dino oils the opposite.

That being said, I'd just buy the same oil across the board. I've been using M1 0W40.

Don't mean to thread jack but, what about changing to the heavier oil around the summertime when it gets real hot, and then back to a say 0-40 for the cooler parts of the year? Use to do this on recommendations from a mechanic friend. But this was not on Porshes by the way.

Thanks guys.
JFP's logic of different additives not having a cumulative effect on oil weight makes sense, and since there is no telling if Mobil 1 is consistent in using the same additives for viscosity throughout their line...
I'll steer clear of cocktails and boatdrinks for my car and just stick to a single malt beverage, paying extra and waiting a bit to get a good marque (Castrol Syntec) if I have the time, sipping the local (M1) brew if the time window is tight.