can i mix oil to make my own viscosity?

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Feb 1, 2003
palm beach
this is one of those stupid things where i am 99% sure i am right, but little nagging voice in my head makes me ask.

i bought a tonne of oil from target, the havoline 64 cent/qt deal.

they didnt have 10w30 or 10w40 which i use interchangably.

i bought a tonne of 5w30's and a tonne of 20w50's.

what i plan on doing is using 3 quarts of 5w30 and 1 quart of 20w50 for each oil change, this *^*should*^* net me approximatly a 9w35 correct? and this is ok to do!?
I have mixed Mobil1 SS 0W-30 and 0W-40 to bring the vis to around a low 11 while still keeping a 0W oil for winter. I used the typical oil vis info from the product data sheets. I might do it for summer to just get the higher vis, but use a 5W-30 or 10W-30 as the base,since most Mobil1's oils vis are usually arount 9.7-10.1 for their lighter grades.

Search the forum, there has been some discussion on how accurate the blend calculation and the merits of doing your own. Personally, within the same brand and close grades, I think it would be pretty accurate. Mobil1 has blessed the blending.
They just don't tell you how to do it.

[ March 04, 2003, 01:44 AM: Message edited by: Fillherup ]
It works fine for me, I've been mixing castrol 10w-30 and 20w-50 for years for my Toyota which calls for 20W-40 (Hasn't been avaialable for years.) The resultant viscosity won't be the exact average of the two but defintely will be somewhere in between. Good enuff for me.
Works for me too, I've been doing it for years.
I normally use the recommended wts while under warranty and then go thicker after about 60k miles, I usually mix wts to do that....
Actually the only vehicle I ever had that did have an early engine death (at 107k miles) I had stayed with 10w/30 the whole time...I don't blame the oil though, I believe it was a temp issue, it's a long story...
bottom line, I personally don't have a problem with mixing oil wts, oil company folks have told me they are compatable, and that the math for the wt average is not exact but close enough
You can't numerically get the numbers with a weighted average, but you probably are not far off. I woudn't have a problem with it since it is the same brand.
One thing to consider if you are going to do oil analysis is that you won't know if the viscosity is close to what it started with since you will only be estimating the effects on viscosity.

There is now 15W40 now (finally) if that helps. I know it's available in Tech2000 and maybe a few others. That's what I am starting to switch to in some of my cars incl my Toy. In fact I'd like to see the shelves move more from 10W40 towards 15W40. My opinion is that it fits nicely and more logically between 10W30 and 20W50.

"I've been mixing castrol 10w-30 and 20w-50 for years for my Toyota which calls for 20W-40 (Hasn't been avaialable for years.)"
Isn't that 15W-40 a heavy duty diesel motor oil?...API CH-4 or CI-4? If it's also API SL, it'll be OK for a gasoline engine, maybe work quite well.

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