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06-18-2010, 10:23 PM
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CC Member
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Join Date: Jun 2006
Location: Merced,
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Cobra Make, Engine: West Coast-Ford Performance Solutions 533 BB
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Oil viscosity mixing?? Help!
Sorry for a last minute question, but the wiffie and I are headed out for a long trip at 6 a.m. and I need to top off my oil a bit. Trouble is I found 4 containers of Pennzoil 10/30 and 3 containers of Castrol 20/50 on my oil shelf tonight and so help me I can't remember which I have in the car right now...will I cause problems if I happen to mix the viscosities? (I know you gearheads learned this at age four)
Thanks for any replies
Dirk
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Last edited by DocDirk; 06-18-2010 at 11:44 PM..
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06-19-2010, 12:50 AM
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Shouldn't be a problem at all. If you're going on a raod trip you probably want the 10W-30 oil, so I would top off with that. |
06-19-2010, 07:31 AM
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Thanks Bob!
We're on our way out the door right now and looking forward to a great trip.
Regards,
Dirk
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06-19-2010, 07:36 AM
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I always wondered this also. What happens when you mix different oil weights. Does it create and average of some type? |
06-19-2010, 08:14 AM
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I use Brad Penn oil. They said to mix 10w30 with 20w50 50/50 to get a 15w40 blend.
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 Snakebit
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06-21-2010, 06:12 PM
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Yeah, It averages out. Also when you top off it helps the overall viscosity & lubricity (is that a word?) of the used oil already in the motor.
My wife's daily driver uses a quart every 1000 miles & I don't mind as I know it's getting some fresh oil every 1000 miles. We change the oil every 4 to 5 thousand & the oil still seems to be in pretty good shape.
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06-21-2010, 07:09 PM
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It does not average out, what happens is part of the oil acts like 10w30 and part acts like 20W50. It is not a homgenous mixture. It is not the a huge issue but I would not make a habit of it.
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06-21-2010, 09:03 PM
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Quote:
Originally Posted by madmaxx
It does not average out, what happens is part of the oil acts like 10w30 and part acts like 20W50. It is not a homgenous mixture. It is not the a huge issue but I would not make a habit of it.
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Not an homogenous mixture? 
10w30 mixed with 20/50 would be a "mix" of the two depending on the ratio involved.
Especially after the trashing the oil gets within the engine.
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06-27-2010, 04:34 PM
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The same goes if synthetics were involved,no worries about mixing any of them.
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06-27-2010, 04:49 PM
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Quote:
Originally Posted by madmaxx
It is not a homgenous mixture.
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So you have large pockets of one weight floating around with large pockets of the other weight floating around, both in different locations within the engine. Kind of analogous to the gulf oil spill. 
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06-27-2010, 07:09 PM
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No you have some molecules of 10W30 floating around and some of 20W50 or whatever the you are mixing. How hard is it to get the correct viscosity. You might want to visit bob the oil guy website for details.
I have to admit it amazes me that we on this forumn know more than the engineers who design engines. Some have to make a special mixture, WTF.
Go to Joe Gibbs racing oil website under muscle cars and if you went buy his recommendation on temperature and horsepower you would be using 5W20!!!!
Remember your oil pump is a positive displacement pump, regardless of the viscosity it pumps the same amount, you see higher oil pressure with thicker oil because it does not flow as easy NOT BECAUSE IT IS PUMPING MORE. In fact thicker oil does not carry the heat away as efficient, does not lubricate in many cases as well, for example the oil will just overflow your rocker arms instead of penetrating throught the bearings and the oil hole for the valve stem.
99% of this forumn should be using nothing but 10W30 or in some cases 5W30.
Quote:
Originally Posted by CobraEd
So you have large pockets of one weight floating around with large pockets of the other weight floating around, both in different locations within the engine. Kind of analogous to the gulf oil spill.
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06-28-2010, 09:22 AM
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The 10W30 mix with 20w50 was from the Brad Penn web site. My engine builder recommended 15w40 oil.
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Snakebit
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07-04-2010, 10:53 AM
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In practical application in my old German v8s which are very sensitive to viscosity changes I have been known to mix a custom blend of weights.
Just another tool at your disposal
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Steve SPF 2734 MK3 / Brock Coupe #54- panavia.com
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