Re: BMW Synthetic Oil

["Brant M. Miller" <brant@xxxxxxxxxxxxxx>]

Aaron -

You have asked two very separate questions.  I'll share
what little I know about each:

1.  Oil change intervals are determined more by the life
of the additives than by the base stock.  So unless
you believe that the additives are better in the
synthetic oil, you should continue with your 3000 mi
change interval.  If you would like to be more scientific,
take oil samples and have them analyzed.  Find a
good tribologist and determine what needs to be
measured (it is NOT viscosity as some
peddlers of synthetic oils seem to suggest).  Be
sure to include Base Number in the analysis.  It
is probably the best determinant of need
to change oil.

2.  The "5W" part of the viscosity range has the
most effect on fuel economy.  I doubt you would
see a difference in fuel economy between
5W30 and 5W40, although the 5W40 will be
more viscous at normal operating temperature.
The greater viscosity at normal op temp is why
BMW selected 5W40.

3.  An additional factor to consider re the change
interval:  Older oils used viscosity stabilizers
to achieve multi-grade viscosities.  They broke
down in time, leaving the oil too "thin" at
normal operating temperature.  In some cars, that
would show up as an increase in oil consumption
toward the end of the change interval.  Synthetics
are much less reliant on viscosity stabilizers - =

I've heard that some do not use them at all.
But perhaps some 5W40s are more reliant
on vis stabilizers than some 5W30s?   Have a
chat with your friendly tribologist.

Brant

>Date: Fri, 11 Jun 1999 04:49:47 -0700 (PDT)
From: Aaron C Liu <aaronliu@domain.elided>
Subject: BMW Synthetic Oil

I recently switch to Original BMW High Performance
Synthetic Oil and it is rate as 5W-40.
I like to know how often should I change the oil.
Can somebody tell me what's the difference between the 5W-30 and 5W-40?
Aaron C. Liu            North Seattle Community College
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