How to calculate the viscosity of oil when mixing oils?

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Jun 20, 2002
Montgomery, Al
When I use M1 10/30 oil I get a lot more engine (98 LS1) valve type noises plus there is a lot of residue on the magnetic tip of the oil plug.
This last oil change I went with MaxLife 10/40 and there were no abnormal engine noises and very little residue on the magnetic tip of the oil plug.
Well I decide to try Mobil1 again (the new formula). I knew the 10/30 was not going to work without mixing it with some 15/50. So this is how I determined the amount to mix in order to get a viscosity similar to the Max Life 10/40.
Using the viscosity Kinematic, cSt @ 100°C listed below I determined that mixing 3.5qts of 10/30 with 2.5qts of 15/50 would yield a viscosity Kinematic, cSt @ 100°C of ~13 and
2.5qts of 10/30 mixed with 3.5qts of 15/50 would be ~14.2.
I added the viscosity Kinematic of each oil in the amounts listed above and divided by the total amount of oil mixed (6qts). I averaged them together.

Is this a correct way calculate viscosity when mixing oil?

Kinematic, cSt @ 100°C

Mobil1 10/30 = 9.8
Mobil1 15/50 = 17.4
Max Life 10/40 = ~14
Total capacity of the engine = 6qts

The above is being used in a 98 Z28 with the LS1 engine. 37k miles


It's directly linear/proportional.

ie (to pick an easy example) mixing 2.5 quarts of 5w-30 w/ 2.5 of 10w-40 will yield the behaviour of a 7.5w-35 (if there was such a grade).

thats right I cant edit, I am just so used to be typing away and police up unessecary things aferwards..

Anyway its clear you didnt need my example, its clear you understand averages.

If you're hooked on Mobil 1 for sure, why not try their 0w40 viscosity? Or, try Redline's 10w40, or Schaeffer 15w40. While mixing viscosities is not really harmful, you're better off simply getting the viscosity you desire right off the shelf instead of trying to mix it up. I know it seems to be working for some people, but I'd really like to see some oil analysis results on these mixed oils to know what's going on in there.
Dominic – I went back to Mobil1 because I wanted to use synthetic oil and to see if the engine noise I had in the past was related to the product or to the viscosity. FWIW with the mixture (vis=~13) I have little to no abnormal engine sounds. However there is more noise now than with the MaxLife 10W40.

Patman – I wish I had tried out their 0w40 viscosity oil (Vis=14.4). Unfortunately l did my research after the change to mixed Mobil1. Next time I will try the M1 0W40 or maybe Redline's 10w40.



Originally posted by sprintman:
How about Redlines new 5W40?? I have the specs if anybodys interested (it's still not on Redlines web site).

Tell us more! I wonder if it's going to be sold worldwide though?
Do the oils actually mix or are you just getting 3/4 molecules of 10-30 and 1/4 molecules of 15-50? ie. both oils are there and the 10-30 will boil off at the same rate as it would if you just ran straight 5-30.

Why do you feel the need to mix anyway? Is 10-30 too thin for you?
Temporary mixing within the same viscosity is ok for emergency purposes, but when you mix across viscosities, you're dealing with different levels of additives and different base types and base viscosites.

Bottom line is, unless you do a virgin oil analysis, you don't know what you're getting.

Go with the Delvac 1 5W40 if you want a higher vis.
Redline 5W40 specs

V @ 100C 15.1 cSt
V @ 40C 94 cSt
VI 170
CCS Viscosity 55 poise@-30C
HTHS 4.6cP @ 150C ASTM D4741 (good figure that me thinks)
Pour point -49 F
Flash 480 F
NOACK 1hr @ 482 F (250C) 6% loss.
Dr. T – I mixed the oil because I wanted Mobil 1 oil that was thicker than the 10W30 but thinner than 15W50. My motor knocked and made a lot of abnormal internal noises with the M1 10W30 oil.

MolaKule – Where do you get Delvac 1 5W40. Where can I find the specs. On this oil?

Sprintman – Those Redline specs look good. Thanks for the info.


Even with the mix of M1 10W30 and M1 15W50 the motor is much nosier (knocking sound) than with the Max Life 10W40. For what ever reason my LS1 does not like Mobil1 so later this week I will drain out the $30 worth of Mobil1 (use it for the lawn mower) and replace it with what I know works the Max Life 10W40.

FWIW I just drained out the Mobil1 and put the Max Life 10W40 back in. Within 2 miles of driving the engine is quiet as an LS1 can be.

John, was this Tri-Synthetic? I've found cars to be especially noisy with that stuff in their crankcases, my Honda Civic included. I'd expect Super-Syn to be better ... but I'd like some more evidence.

Because of the vast difference in engine noise, a buddy of mine quit using Mobil 1 and switched all of his vehicles over to Max-Life. I just don't know if some of the last batches he bought are as good as the earlier stuff. I bought my Dad some early 10W30 Max-Life a year or more ago because a local Wal-Mart was selling 5qt. jugs for $5.

I haven't tried Max-Life (which has recently been reformulated, by the way) in my Civic but when I switched from Mobil 1 & Valvoline Synpower to Red Line, there was a WORLD of difference. The engine, which WAS noisy (especially in the cold), quieted right down and oil consumption which had begun to become bothersome reversed itself and is back to (essentially) ZERO.

--- Bror Jace

Originally posted by sprintman:
which viscosity Redline?

Bror has run both the 5w30 and 10w30 Redline. I remember him saying his MPG was a good bit better with the 5w30 too.
"I'd expect Super-Syn to be better ... but I'd like some more evidence"

I have no evidence but I have some testimony

The Wifes 02 Cinco Wagon with 7K now runs as quiet as it did with the Pennzoil I used while running the motor in after changing to the Supersyn 10/30. Although not a comparison to the Trysyn, I have read that some have said their motors are more quiet with Dino oil than the SS is why I posted. I never could see the reasoning there so it did not scare me away,speaking of scared,it's almost Halloween
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