prophet@umcp-cs.UUCP (Dennis Gibbs) (09/27/84)
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Several people have recently expressed interest in exactly what is a multi-
viscosity oil. The answer is actually quite simple.
A multi-viscosity oil, such as a 10W-30 oil, means that at zero degrees
fahrenheit, the oil will have the same thickness as a straight weight 10W.
It also means that at a high temperature (I think the standard is 210
degrees) the oil will have the same viscosity as a straight 30 weight.
This does NOT mean that the oil doesn't get thicker as it gets colder, it
just means that its viscosity value changes from a 30 weight at operating
temperature to a 10W weight at zero degrees fahrenheit.
So, in essence, what is happening is that you are getting the high-temp-
erature protection of a 30 weight, and the low temperature protection of a
10W weight. Both a straight 10W weight and a straight 30 weight get thicker
when it gets cold, but a 10W weight is always thinner than a 30 weight.
Also, there is a common misconception that the heavier the oil, the better
the oil protects. This isn't necessarily true. What is important is how
much temperature and pressure the lubricant can take without oxidizing ex-
cessively. This is where Synthetic lubricants excel.
Dennis
--
Call-Me: Dennis Gibbs, Univ. of Md. Comp. Sci. Center.
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BITNET: GIBBS@UMDB
ARPA: prophet@Marylandcjohnson@zehntel.UUCP (Chris Johnson) (09/27/84)
The standard *high* temp is 220 deg. f. This is important because one of
the major problems with Multi-Vis oils is that as the tempurature varies
*outside* of their intended range they tend to break down faster than
the corresponding single viscocity oil (ie. 10w30 is worse at 230 degrees
than straight 30 weight.) In a water cooled engine this isn't that important
because the engine basically won't heat up over 220 degrees, but in an air
cooled engine temps can reach up to 250 degrees in worst cases. This is
especially important because as the oils viscosity drops the oil pump
doesn't work as well. I owned a VW bug with a *very* tweeked engine and
found that 30 wt oil heated to a point that the engine would regularly
vapor lock due the temp in the engine compartment (it is especially
strange to have dual carbs with one vapor locked). I switched to
50 wt racing oil (Valvoline) and the problem disapeared.
Analysis:
If you own a water colled engine use a multi viscosity oil if your
climate is severe enough to require it. (30 wt oil is *very* thick at
0 degrees) If your climate is constantly 40+ degrees use what you want.
Don't use single viscosity 10wt or 20wt oil unless you live in -20 degree
weather AND your car never heats up to operating temp.(ie. you use it
for short drives only.)
If you own a air colled engine use the highest viscosity oil you
can *for your climate*. Change your oil's weight if the weather cools during
winter.
DON'T mix oils of different weights. The thicker oil will sink to
the bottom of the oil pan and just sit there, inhibiting cooling by
providing an insulating layer (esp air cooled).
DON'T mix synthetic oils and standard oils for similar reasons.
Slipperily yours,
Chris Johnsonmarcus@pyuxt.UUCP (M. G. Hand) (09/29/84)
At one point I used to use BP's VPF-7 which I found excellent. It was
a peculiar oil in that it posessed both low viscosity and high surface
tension, normally counter-active attributes. On new(ish) engines in cold
weather, it provided excellent protection because the parts tended to remain
covered and the oilways full when the engine cooled, but the stuff also
sloshed around the engine straight away ('cos it didn't have to heat up
as much as conventional lubricants). I would hesitate to use it on older
engines with severely worn oilways, though.
Anyone know where this is available, I haven't been able to find any
in NJ area?
Marcus Hand (pyuxt!marcus)gino@voder.UUCP (Gino Bloch) (10/01/84)
>> DON'T mix oils of different weights. The thicker oil will sink to >> the bottom of the oil pan and just sit there, inhibiting cooling by >> providing an insulating layer (esp air cooled). >> Chris Johnson Do you really believe that? I don't. Do you know where the oil pump inlet is? Are you sure that thicker oil is heavier than thinner? That synthetic is heavier than natural? That oil & oil don't mix? Is your other advice as good? -- Gene E. Bloch (...!nsc!voder!gino)