Mixing different viscosity oils

Messages
11,006
Location
Canberra ACT Australia
If you mix say 2 quarts 5W30 to 2 quarts 10W40 do you have exactly 4 quarts 7.5W35 or is there more too it? Are the results the same for mixing any viscosity as long as the brand/type are the same? Tks...s
 
Messages
1,856
Location
PA
I hear they don't mix well, and I wouldn't mix viscocities unless my life depended on it (i.e. getting out of the desert, all I can find is 20W50, I'm leaking oil bad and need something now). Try to stick with one weight or another - what are you trying to accomplish by mixing weights?
 
Messages
5,785
Location
Dixie
Sprintman, The different SAE grades represent basestocks having varying molecular weights. If you were to blend them together the viscosity characteristics would probably not be determined by a simple law of mixtures. For example, mixing together equal parts 5w-30 and 20w-50 would not necessarily produce a 12w-40, or something like that. A good analogy would be mixing small and large marbles - you don't end up with medium sized marbles as a result. If you were to heat this mixture up, the lighter, more volatile portions of the mixture - from the 5w-30 - would still tend to evaporate. Likewise, if you cooled this mixture down, the heavier molecular weight contribution of the 20w-50 would probably cause rapid thickening. In other words, they are being physically combined as opposed to chemically combined. So you could in fact end up with the worse characteristics of both these grades in terms of extreme temp properties.
 

MolaKule

Staff member
Messages
21,706
Location
Iowegia - USA
In usual practice, you are adding the same base oil and replenishing the additives, and bringing the viscosity back to original.
 
Messages
885
Location
North Carolina
Here I go, walking upstream again... I will use 10w/30 while my vehicle is under warranty and after about 50k miles I start using thicker oil. Since I already have 10w/30 and 20w/50 both on hand, I mix them about half/half. My 99 S10 ZR2 truck is getting better mileage on the half 10w/30, half 20w/50 mix oil...not sure how to explain that though. I have asked oil mfgrs about mixing oil wts and they all said the same thing, they make most any wt I could want, but mixing won't hurt. As far as the resulting viscosity value, it may not be an exact mathematical average but is close enough. You could also say that you get the best characteristics of both oil wts...seems to be working for me, has for years. Far as I'm concerned the xw/30 oils have been a CAFE requirement, and is a marginal wt oil. I am more concerned with engine longevity ..but to each his own I hope nobody on this board is in New Orleans, good luck to them if they are..
 
Messages
622
Location
42.4N 85.7W
I've been considering this myself. I'm using Amsoil 5/30 in my car, but the wife's car is still under warranty, an Mobil1 is easier to obtain than Amsoil, but their 5/30 and 10/30 are thinner than I'd like. Wifey's car has a 6qt sump and I was considering 5qts 5/30 and 1qt 15/50. Just to add confusion to my decisions, she got a new job two miles from home and I don't feel comfortable repeating the 'Aunt Minnie' test.
 
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