TOL-1 Mixing different viscosity oils?

[Loobed]

I am Thinking Out Load (TOL)

What really happens to the oils viscosity when you mix oils that are 2 different viscosities? Figure the oils are the same brand (Mobil, Pennzoil, etc), and type (syn/dino).

I read something in a car magazine about 20+ years ago that basically said if you mix equal parts of 30 weight oil and a 50 weight oil, you do not get a 40 weight oil. You get an oil that is 50 percent of each grade. I am not an oil chemist, but I can see where this might be possible. Each multigrade oil has a different viscosity improver that thickens the oil to a specific point. Mixing one VI that thickens oil to a 30 weight with another VI that thickens oil to a 50 weight shouldn't change the VI in each oil to a 40 weight VI.

If my engine is designed to use 6 quarts of 5w30 oil, what would happen if I used 3 quarts of 5W-20 and 3 quarts of 5W-40? Would the 40 weight oil prevent the 20 weight oil from flowing into tight clearances, and would the 20 weight wash the 40 weight off the cam lodes and cylinder walls?

I know it is easier to use the right oil, but I have some older oil sitting around that I would like to use. It's those [censored] oil rebates that cause these problems.

 

[Steve S]

There are no clearances in an engine where the 40 visc oil will not flow through [or fit], The oils will blend together and you will get what ever the mix turns into and it will be no biggie. i jut did an oil change where there was 5 qts of Pennzoil oil syn and 1 qt of Valvoline syn . I am not worried in the least. When at work the parts guy would come out into the shop with a cart that had 3 different brands of oil totalling 13 gallons of oil and the filters.I think he cleaned up all the odds and ends of HDEOs in the parts room and say Ron [the service manager ] wants you to change the oil in [for example] semi #8 [there was 3]. That truck was old when I started working there and was older 17.5 years later when I left the company.