4qts 10-30 and 1 qt 20-50

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Jul 10, 2002
Iwas wondering if mixing 4 qts of 10-30 and 1 qt of 20-50 castrol woud have any harm during the summer months in Texas?
What do you hope to acheive by doing this? It probably won't hurt, but if you just want a slightly thicker oil, why not use a 15w40 oil instead?
I mix oil weights too, same idea. I use some thicker oil during the summer than I do during the winter, and I use thicker oil on older vehicles. 15w/40 would be a good alternative just like Patman suggested. The reason I mix wts is partly because I always have a lot of 10w/30 on hand because of another newer vehicle anyway, For the vehicles with higher mileage I just mix in the 20w/50 with the 10w/30 that I already have at the house. Rando [ July 11, 2002, 06:50 AM: Message edited by: ZR2RANDO ]
I would only do if I was on an Arizona road, I was a quart or better low and that's the only oil I had in the trunk. [Frown] Each grade of oil has a different level of viscosity index improvers, for starters. [Smile]
As said, best would just be to change to 15w40. But at least you should stick to ONE brand when mixing.
There is not a problem with mixing oil wts folks, the result of mixing 2 different wt oils may not be an EXACT mathematical average, but it WON'T be far from it. There is more VII in the large spread viscosity oils is true, but oils like 10w/30 and 20w/50 do not need much of it anyway so it does not really come into play any more than using the unmixed oils. The API standards are required for a reason, so the oil can meet those standards. If there are any brand incompatibility issues then that brand had better state in LARGE letters NOT to mix the oil with any others. We all (here on Bob's and Edmunds pages) have our favorite brand for whatever reason and try to stick with it until we learn of something better (me too,I use Havoline/Chevron and I do mix wts), but don't be so naive as to think the majority of drivers care about it as much as we do. The majority of people change the oil wherever it is on sale and probably could not tell you the brand or wt of oil in the car at any given time. Most people are content to stick with the wt oil that the mfgr recommends without thinking about WHY they recommend that weight, or taking into account the age of the engine. Some of us are just more concerned with our own priorities rather than the mfgr's priority...In particular, I am more concerned with engine wear than I am about a fraction better gas mileage. Mixing wts is one way to get thicker oil which works for me. Buying a 15w/40 is another way of doing the same thing, but I already have the other oils on hand so I mix. Keep in mind also that all the oil brands do not always make their own base oils and additives, they buy from and sell to other companies too. When you really check into it you may be surprised when you find out who makes whoever elses oils. Marketing a product can make as much or more $$$$ than actually making a good product unfortunately. As always, we all have our own way of doing things, and these posts are a good way of seeing other points of view. What I do works for me and what someone else does works for them, interesting concept huh? see ya Rando [ July 11, 2002, 06:55 PM: Message edited by: ZR2RANDO ]
I am very comfortable with mixing also. Right now I have 3 qts. of 10W-30 and 1 qt 15W-50 Mobil 1. I am real fanatical about checking milage and I really can not detect a noticable difference. I am sure there is some but as I average 28-32 in the mixed driving that I do its too close to call. Next month I will be going back down to N.C. Its a 400 mile drive and I should be able to detect a differenc. And with Mobil being on the thin side of 30 wt-I'm very comfortable with a couple extra points on the viscosity scale. Now I could be wrong but it does appear that my temperature has very slightly dropped.
I don't see any problem mixing oils, but I don't think that you can easily determine the viscosity of the mix. I don't remember where I read it, but in mixing fork oils, which are straight grade, a 10 and a 20 do not give you 15. More like 18. Throw multi weights in and who knows? The SAE grade system is an arbitrary scale imposed on something that behaves logarithmically.
If you compare several different brands of oil you will see that even the same wt oils will vary in the viscosity values (and other physical properties)(Havoline and Chevron are showing the same numbers by the way..) between each other, it is a range, not an exact number. Five different 10w/30 oils will be close to the same viscosity at any given temperature, but not necessarily the same. Like I said above mixing 2 oil wts may not give a mathematical average (linear) but it will accomplish what you are trying to do..(close enough for government work if you know what I mean he he!!!) The viscosity change as a function of temperature is not linear either. Don't get hung up on trying to have an exact viscosity, it would make your head spin. It has been stated (several times) that the mobil oil is thinner than comparable wt other brand oils right? Some are thinner and some are thicker, we all have our favorites. [ July 11, 2002, 06:54 PM: Message edited by: ZR2RANDO ]
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