Mixing oils of different viscosity
 

If you mix equal parts 5w-30 and 15w-50, is the result 10w-40? Or is there some more subtle non-linearity? Would they blend together properly, or would the heavier oil just sink to the bottom? Is there any reason to avoid mixing oils of different viscosity, assuming everthing else is the same (brand, synthetic, grade, etc).

While you're not hurting anything, I don't think the oil math works like that. I've been doing it for years - during the summer, I would change the oil with 20w50, and add a couple quarts of straight 60w.

I actually e-mailed Mobil 1 technical support about this since I want to use something thicker than a 10/30 and thinner than a 15/50 in my Audi A4.

Their response supports your assumption. If you mix equal parts of Mobil 1 5/30 and 15/50, you get a 10/40. This is true for Mobil 1, so I can't say if it is the same for conventional motor oils since the additive packages may vary.

For a 911, I use a straight 15/50 Mobil 1 in my 80SC. I'm not sure I would use anything thinner.

Noel

I always use 15-50 in the Carrera, but in the middle of winter add a few quarts of 5-30 and in the middle of summer a few quarts of 20-50.
The main reason I asked was that Audi recommends 10-40 for my A6, but Mobil 1 does not make a 10-40, so I have been blending 5-30 and 15-50, increasing the 5-30 share in winter and increasing the 15-50 share in summer. I wonder if this qualifies as an obsessive-compulsive disorder.
I have noticed that Syntec is now offering blneds of synthetic and non-synthetci oil, though I dont know why one would want to do that, other than a lower price.

I keep telling my wife that it's people who don't obsess about their cars that have the problem. A clasic case of denial, I know.

Noel