Mixing Different Oil Viscosities, Same Brand

[THX1138]

In the daunting process of cleaning out the garage I found a few bottles of Brad Penn 10w40 Partial Synthetic. If I mix these in equal parts with Brad Penn 0w30 Partial Synthetic, will I get 5w35? Or will I just have an oil pan full of two different viscosities? I tried searching for answers on old BITOG forums but other than "Sure, go ahead", I didn't find a reasoned answer. Is there a down side to doing this?

 

[Greasymechtech]

Oils mix.
Blend away. Use up the quarts. Don't worry about.

You'll get a good 5w30ish oil.

 

[HTSS_TR]

You will not get 5W35 because there is no such grade available. You will get thicker xW30 than most on the shelf.

 

[Merkava_4]

Use this oil viscosity calculator to find out what you'll have.

 

[THX1138]

Thanks for the link Merkava. The Widman website has an article on "Oil For Flat Tappet Engines" (Corvairs and others) that is great reading for owners of older cars. Thirty-three pages of detailed analysis of how to choose the correct oil with recommended oils. Great stuff.