Kinda, yes you kinda would. I have been known to need a quart of 10w30....I'll mix the lawn equipments Diesel rated 30w in with the 10w30.....therefore.....not really hurting anything. Although, when I do this......I move the oil change up a little in order to get it right. Ya know, possible additive clashes.
In all practicality........its very unlikely that you'll damage anything.
I sure was long winded for such a short question.
Just like mixin 2-3 Brands of whisky,Burbon you get Lubricated.And yes you can use your own choice of additives & probaly the only damage you would do is to your Brain & Liver, But with the oil you dont get ****ed. Hope this makes sense, pretty hard to put in words because i had a few Whiskys & Beers today so maybe thats upset my weights Bruker at this particular time of day its hard to answer a trick Question.
I always figured it this way. Not saying it's right, just the way I do it. In a multiweight, the front number is the weight of the base oil they use. So that can be averaged. I'll use the example of a quart of 5w30 added to 2 quarts of 10w40. So (10+10+5)/3 = 8.3. So the first number is 8.3. Now this is where it gets interesting. We need to figure the amount of viscosity index improvers in it. So in the 5w30, we have 25. In the 10w40 we have 30. So (30+30+25)/3 = 28.3. Then we add that to the 8.3 to get the second number. So we wind up with 8.3w36.6
It gets really interesting when adding multiweight to straight weight. Say we add a quart of 5w30 to 2 quarts of straight 30 weight. It becomes a multiweight in the end. Watch. (5+30+30)/3 = 21.6 So our first number is 21.6. Next we figure the amount of viscosity index improvers. So, (25+0+0)/3 = 8.3. So, we end up with 21.6w29.93.
Did you ever have to add a quart of oil to your truck and you just didn't have the same weight oil that was in the truck? If you mixed a quart of 5w-30 and a quart of 20w-50 would you get 12.5w-40?
Hard to say. Something in between. I'd guess about a 10w40. The base oils (and I am leaning away from using the "w" numbers as a good guess at the base oil*) would mix with a resulting viscosity slightly below their average. Then the other factors like viscosity index improvers would play upon that. Yeah, probably a 10w40, but it would have a bi-modal (dumbell) distribution of the base oil molecules, which apparently is not desireable given that one of the strong points of synthetics is their very tight oil moledule size distribution (basically all one size).
BTW I did this once. I mixed four 5w30s with two 20w50s trying to simulate a 10w40. When I switched to 10w40 the oil pressure reading was the same as I got with the mix. Much better off mixing, say 10w30 with 10w40 as the base oils will be much closer.
Also, we have to remember technically we can't do math with the grade numbers. They are just categories and of varying width. They could as well have had letters. Would you like some BwF grade oil? For ballpark, get the 100C viscosity of both grades and average it. Should be close enough. 5w30 is about 10.6 cSt at 100C (Maxlife) and 20w50 Maxlife is 17.4 (this data is available at companies' web sites). Average is 14, a solid 40 weight (40 weight ranges 12.5 to <16.3, and Maxlife 10w40 is 13.5). Check out this nice grade chart: http://www.infineum.com/information/viscosity.html
* The European Redline Oil site's FAQs discuss that a straight 30 weight oil can pass the 20w test and so could be called a 20w30. Add some VIIs and you get a 20w50. This implies that a 10w30 and 10w40 very well may have a base oil in the 20 weight range. Problem is, the oil companies will never tell you what the 100c viscosity of the base oil is.