The Stovebolt Site Forums The Shop Area Engine Shop Mixing oil grades

I was just wondering if it matters if a person mixes 5W30 and 10W30 does that just make 7.5W30. I needed to add some oil in my truck which I run 10W30 in but all I had in my shop was 5W30. Should I be concerned or just keep driving?

It shouldn't matter, I do it all the time and my 77 Nova still has an intact original motor. It still has original timing chain and I've never adjusted the lifters or any thing .It nowhas 158,000 miles.

I agree with ya, Killer. There isn't all that much difference between 5w30and 10w30. I sure wouldn't lose any sleep over it.

On the other hand, I don't really care for multi-viscosity oils. They have always seemd to take on the characteristics of the lower viscosity rating more than the higher; that is, they seem to run a lot thinner and in my mind that's not a real good thing when it's hot. Cold weather is a different story, but since spring is just starting to sprung here, I prefer 30 or 40 weight oil. That's just one guy's opinion, tho.

A good explination of multi-viscosity oil:
http://www.autoeducation.com/autoshop101/oil-change.htm

Polymers are added to an oil that meets the low temperature requirements to keep it from thinning out. This allows it to meet the high temperature requirements as well. A 5W30 oil is really a 5 weight oil with polymers added to make it behave like a 30W oil at high temperatures. Mixing a 5W30 with a 10W30 50:50 probably won't get you a "7.5W30" oil. Having said this, I wouldn't worry about the occasional mixing of 10W30 oil with 5W30. That second number is the most important one once the weather starts to warm up and you don't have to worry so much about the oil flowing like mollasses in winter. Since both oils behave like 30 weight at high temps, you should be fine, though with the addition of the 10W30 (which has fewer/different polymers than the 5W30), it might be slightly thiner at high temp. Say 28 weight oil??