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Re: Engine Oils and Viscosity

To: bobmgtd@home.com
Subject: Re: Engine Oils and Viscosity
From: "Richard Lindsay" <rolindsay@dgrc.com>
Date: Tue, 29 May 2001 21:04:43 -0500 (CDT)
Hi Bob,
   I use Castrol GTX in 20W50 grade.  However, thicker does 
not automatically mean better -- like it used to.  This is 
a topic of due consideration.  I have researched oils and how 
(and why)they work and would enjoy jumping in the mix.
   Given the engines in our LBCs, 20W50 is probably best.  
Multi-viscosity oils obviously have properties unlike single 
viscosity oil -- which isn't really just one viscosity!  Oil, 
like many liquids, thins or becomes less viscous as temperature 
increases.  And what is the right viscosity for a COLD engine 
is different from the needs of a HOT engine.  Leaving out 
specific temperatures (because I can't remember them), 20W50 
oil has the viscosity of cold 20 weight oil -- when cold and hot 
50 weight oil -- when hot.  That is, it is thinner than 50 
weight when cold for better circulation when cold and thicker 
than 20 weight oil when hot for better thin film tear strength.  
But what do all these words mean?

Short answer:  Use 20W50 in your LBCs

Long answer:  The meaning of 20 and 50 are given above.  The 
  " W " is/was for "winter."  20W50 weight multi-viscosity oils 
  are made by adding coil-shaped polymers to pure lighter weight 
  weight oil.  The polymer coils unwind as they are heated and 
  cause an apparent thickening of the oil.  This increased 
  viscosity improves the tear strength of the thin film covering 
  the bearings and preventing metal-to-metal contact.  Thicker 
  oil also doesn't flow through narrow spaces as quickly helping 
  to keep oil pressure higher with worn bearings <with excessive 
  clearance>.  This is of course, is a double edged sword as one 
  of oil's main uses is to cool the bearings by flowing the hear 
  away in the oil.  Thicker oil cannot carry the heat away as 
  quickly.  So what is the logical answer?  
  (1) Shoot Rick.  He's on one of his techno-posts.  or 
  (2) for a healthy engine, use a multi-viscosity oil with as 
  small a viscosity range as possible.  Why?  Because, although 
  the polymers thicken the oil they provide NO LUBRICATION and 
  in fact, their addition dilutes the lubricating properties of 
  the oil!

   I like being a physicist.

best,

rick

> I've used a number of different brands of 20W50 in my MGs, and 
> I swear some are thicker than others. I just tried Pennzoil 
> and it seems the thinest.  Quaker State gets my vote for the 
> thickest. This is based on my totally subjective impression of 
> pouring rate, oil consumption and pressure reading on the gauge. 
> I just wondered if anyone else has noticed a difference.
> 
> BTW: Does anyone have an opinion on which brand is best?
> 
> Bob Donahue (Still stuck in the '50s)

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