Hello;  During the year 1984, the company I work (ed) for conducted this
experiment to try to find out if synthetic oils could help an engine last
longer, run more efficiently, reduce costs.  We bought two identical 450hp
Caterpillar 3408 engines and installed them in identical Hayes HDX off-road
logging trucks.  One used Amsoil 15w-50, the other Chevron Delo-400 15w-40.
 Records were kept on fuel consumption, hours used, weight and miles, as
well as anecdotal comments from the operators, and mechanics.  Both truck
drivers were well known for their care and attention of the truck.  At this
time it was common for an engine to last about 18 months, we were using
Detroit Diesels, and switching over to Cats.  The engine with the synthetic
oil lasted until 1993, and could have gone another 3 months perhaps, but we
were eager to strip it down and have a look inside.  The other engine
lasted until 1990, and was stripped and examined before being sent out for
core.  Photos were compared, both engines failed for the same reason, spun
bearings, rod bearings.  They had within 2500 hrs. of each other.  The
conclusion was that the driver was the big variable in the equation, not
the oil.  Bob could make an engine last longer than Mike could.  Checking
through the records showed that Bob would go more miles with less weight
than Mike.  It ended up that both engines did almost as much work over the
years, and the experiment was dropped.  The extra cost of the synthetic oil
was the deciding factor.

I conducted my own experiment around this time, when I installed a new
engine in my wife's rabbit.  I used synthetic oil exclusively in it for
186,000 kms.  The conditions of this test were a little unfair in that I
went from a 1500cc Diesel to a 1600cc Diesel. From a 1978 engine to an 84
engine.  Of course the 1600 gave more power, better economy (57mpg vs. 51
mpg), and was a better all around engine.  The reason I cut the test off at
186,000 kms was because that was how long the first engine lasted.  The
first engine died because of ring/bore wear, the second engine never did
die, I sold it at 396,000 kms.  The new owner got rear ended within 3000
kms, and wrote it off.  Not as conclusive as the first one.

I use 15w-40 oil in my 87 GL Vanagon and my wife's 85 BMW 535i, and change
the oil in both of them when the service indicator in the BMW tells me to,
as both vehicles get the same amount of miles (kms) on them. Larry Carter