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Re: [Shop-talk] Oil grade selection

To: PG <>
Subject: Re: [Shop-talk] Oil grade selection
From: "Randall" <>
Date: Tue, 21 Oct 2008 23:42:03 -0700
> When I got my new 2003 GMC 1/2 ton truck, the owners manual 
> recommended 5W30 oil but said 10W30 was OK if you couldn't 
> find or didn't want to run 5W.  Nearly 40 years of driving 
> various cars and motorcycles makes me inherently distrustful 
> of such light oil in an I/C engine, and I have run what ever 
> 10W30 is on sale (by the case) in my truck and my wife's 
> Subaru, changed every 3K miles, and using a good NAPA filter. 
>  I think, as you do, that the 5W oil is recommended strictly 
> from a gas mileage standpoint.

What I find ironic about this is that 5W30 is never thinner (less viscous)
than 10W30 is at normal operating temperature.  It's the second number that
specifies the oil's viscosity at 100C, which is pretty close to what the
temperature should be in the bearings when the engine is warmed up; unless
it's really cold outside.  And even "multi-weight" oils get thicker as they
get colder, they just don't do it as fast as single-weight oils.

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