kent@decwrl.UUCP (Chris Kent) (08/12/83)
All right, all you net.auto techies, here's another one for you. What do the rating numbers on oil mean? What really is the difference between 10W30, 10W40, 20W50, and SAE 30? Why should I use one instead of the other? Etc, etc. I recently had to replace the engine in my car (ugh), and the dealer told me to use 20W50. I have always used 10W40 in the past, and this sparked my interest. Cheers, Chris Kent Digital Equipment Western Research Lab
ark@rabbit.UUCP (08/13/83)
I'm not completely sure about this, but I believe that oil rating numbers are a measure of viscosity. The higher the number, the gooier the oil. The numbers are measured by timing how long it takes a standard volume to flow out of a standard sized hole at a standard pressure and temperature. For instance, the SAE 90 oil used to lubricate the gearboxes of some marine outboard motors has approximately the consistency of honey. For use in an automobile engine, there's a tradeoff. The heavier the oil, the better a job it will do of protecting your engine from wear, but the more energy will be lost sloshing the oil around. Manufacturers tend to recommend a particular grade as what they believe to be the best tradeoff for their engines. Ordinary oil's viscosity changes with temperature -- the colder, the gooier. I guess the rating numbers take this into account. Thus, SAE 30 oil will pour readily in summer but be quite sludgy in winter. This sludginess would make a car using it quite hard to start, because the starter has to slosh all that heavy stuff around to get the engine going. Thus, many modern oils are formulated (don't ask me how!) so as to have less of a viscosity change with temperature. Their rating has two numbers separated by a W. The W stands for "winter": a 10W40 oil normally behaves as SAE 40, but in winter its viscosity stays as low as a straight SAE 10 would. These multigrade oils are therefore to be much preferred if you ever have to start your car in cold weather. Incidentally, every car I've ever owned or driven extensively has had 10W40 recommended by its manufacturer. Most "premium" oils such as Exxon Uniflo, Castrol GTX, Mobil 1 (an incredibly expensive synthetic) are rated 10W40.
toma@tekchips.UUCP (08/15/83)
It used to be that single weight oils (such as 20W-20, 30, or 40) had better high temperature performance (they would not decompose) than the multi- viscosity oils (10W-30, 10W-40, 20W-50). I don't know if that is still true. I've had 2 watercooled VW's and an Audi, and they all specify 20W-50 at temperatures above freezing. In particular, they don't recommend 10W-40 above 60 degrees. Here in Oregon, I have used Castrol GTX 20W-50 year around* with no problems. The Audi has 75,000 miles with no engine work (except for warrantee valve seal replacement) and my old Rabbit had 65,000 miles with no engine work when I sold it (it never used oil). A few years ago the subject came up here on oil grades for VW's and I was supprised to discover that everyone but me was using 10W-40. One trouble with 20W-50 is that it is harder to find since most gas stations don't have it. I buy it by the case on sale at a local auto supply store. Tom Almy * Oil and filter changed every 6,000 miles. Cars are garaged.
heliotis@rochester.UUCP (Jim Heliotis) (08/16/83)
Yes, but what does the "W" mean? Is there something different between, say, 20W and 20? Jim Heliotis
res@ihuxn.UUCP (08/17/83)
With reference to the difference between, say, 20 and 20W oil: There are two temperatures at which the viscosity of motor oil is measured. If memory serves me right (flames to /dev/null if not!) these temperatures are 0 degrees F and 100 degrees F (the latter temperature I am unsure of ... read these two temperatures as "cold" and "hot"). Hence, 20W oil has 20-weight viscosity at the lower temperature, while 20 (no W) oil has 20-weight viscosity at the higher temperature. Further, 10W40 oil has 10-weight viscosity at the lower temperature, and 40-weight viscosity at the higher temperature. As to the significance of 10W30 vs. 10W40 vs. 20W50 etc. The higher the number value (the weight) of the oil the thicker it is. Hence, 20W50 is thicker at all temperatures than 10W40. For operation of today's high-engine-temperature cars it would appear that 20W50, then 10W40, then 10W30 would be the order of preference ... however, for a fairly new car (still pretty tight clearances) it will be easier on the starter in the winter if a lighter grade (10Wxx or 5Wxx) oil is used in the northern part of the country. As a side comment ... I once had a friend who had a very old (beater, not antique) car that burned more oil than gas ( :-) for those of you who cannot recognize an innocent exaggeration and would flame me otherwise!). One day he announced that he had solved the problem -- he had switched to something like 90-weight oil. It worked great ... oil consumption went down to zip. Then the year worked its way around to winter and the temperature (in normally tropical Detroit, MI) fell out of the bottom of the thermometer. Have you ever tried to start a car when the oil has the viscosity of rock candy? He took the bus until the middle of March that semester! Rich Strebendt ...!ihnp4!ihuxn!res
cb@umcp-cs.UUCP (08/17/83)
I have seen the W only on low numbers (that is 10W and 20W; there is no 10).
From 20 on they use it without "W" (i.e. 20, 30, 40, 50; there is no 30W, ...).
As far as I can remember I once read that W stands for "winter"
because those oils are used in winter; oils without "W" would be used in
summer.
Actually the "W" does not mean anything per se, it just marks the
thinner oils; kind of draws your attention that you shouldn't use
10W in summer.
Anyone who knows better (I may be wrong)?
Chris
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