Date:         Mon, 29 Apr 1996 12:52:21 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         wabbott@mtest.teradyne.com (William Abbott)
Subject:      Oil, octane

	Fellow asks about changing tranny oil, recomended engine oil,
gas octane. 

	Personally, I change tranny oil on every used car I buy. You 
get a good fill of good stuff, for few dollars, get to see what drains
out. Cars always seem to drive better after you work on 'em anyway. 90W130
sounds neat.
	Things to remember: 
	Use *HYPOID* oil- its hard to find non Hypoid but it can be done
and you don't want it. Our gear boxes have hypoid gears, constant mesh,
and need the oil for 'em. The constant mesh means lower noise but also
means the oil is slid-over by the sliding gear helixes, rather than 
simply being crushed like by straight-cut teeth. 
	Gear oil 'wieght' is determined at a different temperature
(since gear boxes run cooler) than engine oil- so your standard engine
oil is '30' and your standard tranny oil is '90' but they don't appear
all *THAT* different at room temp. They aren't, at room temp.
	You can get multi-viscosity tranny oil and I don't see why
not use the stuff. But how to pick between 75W90 and 90W130 I don't
know.

	Engine oil: 10W40 is my choice. Straight 30 was probably good
advice when St Muir wrote in 1968. Been a bit of progress since then.
The bad rap about multi-viscosity oils is that the additive packages
which cause it to seem to be thicker at higher temperatures (what we want)
get used up as the oil is used. That's correct, and with the 3000 mile
life of the additives, you'll certainly have changed the oil by then
if you want the engine to last. Multi-viscosity oils are good because
in their thin, cold, form, they pump quickly and lubricate fully when
the engine is being started, AND thicken and continue lubricating well
as the engine heats up. Straight 30 doesn't pump so well when cold,
lube as well when hot.

	I've always burned 92 or 89 blends in my cars. Plenty of fine
people burn 87, but I've known of one non-pinging engine which 
failed catestrophicly from heat on a hot day going up hill with a
tank full of 87. Sprayed molten aluminum from the head all over
the undernside cooling tin. Wierd, and creepy. 

	Increased octane is SAID to lower combustion temperature,
which I find believable. At speed, the spark is ADVANCED, because
combustion takes time. The 'Motor' method of determining octane is
to advance the spark until detonation occurs- higher (R+M)/2 means
more resistance to detonation. I don't think detonation is on/off,
but rather some interestingly shaped curve/shape through octane/advance/
mixture/temperature space. I'd be interested to read the results of
anyone with a cylinder heat temp gauge running different octanes.
If I ever get one, I'll post what I see.

	Contrary to an earlier posting, I've heard (here no less) that
increasing Octane typically gives Worse milage- and when I changed from
92 to 89 octane in my Corrado, I seemed to get a bit better milage.
Is it Science? nah.

	Cheers!
	Bill

Democracy is the suspicion that slightly more than half the people are
right, most of the time.