I think a lot of this comes down to good marketing!

I use 20W50 in the summer, 15W40 in spring and fall and 10W40 in the
winter.  Usually use Castrol or Petro Canada non-synthetic.  Two reasons
for non-synthetic.  Both get dirty at the same rate and synthetic is 3
times or more as expensive.  I change my oil on the turn of every season.

The VW manual even states the use of 10W40 or 10W30 for temps between -20
and +10

My feelings are use what the manual calls for and stick to a brand name
oil, change it every 5000km and use a Mann or Bosch filter.

At 03:40 PM 15/10/2000 -0300, Jean-Guy Savoie wrote:
>Hi all
>
>I finally found the reference to 10W-40 not being a good idea : see
>http://members.aol.com/johnbmtl/faqoil.html were this is said, amongst other
>things:
>
>"Multi viscosity oils work like this: Polymers are added to a light base
>(5W, 10W, 20W), which prevent the oil from thinning as much as it warms
>up. At cold temperatures the polymers are coiled up and allow the oil to
>flow as their low numbers indicate. As the oil warms up the polymers begin
>to unwind into long chains that prevent the oil from thinning as much as
>it normally would. The result is that at 100 degrees C the oil has thinned
>only as much as the higher viscosity number indicates. Another way of
>looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that
>will not thin more than a 50 weight would when hot.
>
>Multi viscosity oils are one of the great improvements in oils, but they
>should be chosen wisely. Always use a multi grade with the narrowest span
>of viscosity that is appropriate for the temperatures you are going to
>encounter. In the winter base your decision on the lowest temperature you
>will encounter, in the summer, the highest temperature you expect. The
>polymers can shear and burn forming deposits that can cause ring sticking
>and other problems. 10W-40 and 5W-30 require a lot of polymers (synthetics
>excluded) to achieve that range. This has caused problems in diesel
>engines, but fewer polymers are better for all engines. The wide viscosity
>range oils, in general, are more prone to viscosity and thermal breakdown
>due to the high polymer content. It is the oil that lubricates, not the
>additives. Oils that can do their job with the fewest additives are the
>best.
>
>Very few manufactures recommend 10W-40 any more, and some threaten to void
>warranties if it is used. It was not included in this article for that
>reason. 20W-50 is the same 30 point spread, but because it starts with a
>heavier base it requires less viscosity index improvers (polymers) to do
>the job. AMSOIL can formulate their 10W-30 and 15W-40 with no viscosity
>index improvers but uses some in the 10W-40 and 5W-30. Mobil 1 uses no
>viscosity improvers in their 5W-30, and I assume the new 10W-30. Follow
>your manufacturer's recommendations as to which weights are appropriate
>for your vehicle."
>
>
>For all those interested
>
>JGS
>
>

-- David Marshall - Volkswagen Enthusiast - Quesnel, BC, Canada   --
-- 78 VW Rabbit, 79 VW Iltis, 80 VW Caddy, 84 VW Westfalia        --
-- 85 VW Cabriolet, 87 Audi 5000 Quattro, 88 VW Syncro Double Cab --
-- David's Volkswagen Home Page        http://www.volkswagen.org  --
-- Fast Forward Autobahn Sport Tuning  http://www.fastforward.ca  --
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