Should we be using lighter weight oil?

John Shost j_shost at
Sun Dec 29 15:57:08 EST 2002

[ Picked text/plain from multipart/alternative ]

>>>The first number is the viscosity of the oil at normal temperatures (maybe room temp?)
The first number is obtained at the lowest temperature that the engine is expected to experience. Lower values are tested at lower temperatures. Here’s a link to a chart:
>>>But if you are using a synthetic why not use 0W40 (or 0W50 if available) year round?
The first number relates to the molecule’s size. Small molecules tend to be more volatile. Here’s a link to a volatility chart.
Posting will probably chop link address, you’ll need to paste back together in order to get it to work.--- On Sun 12/29, George Harris < harchris at > wrote:
From: George Harris [mailto: harchris at]To: quattro at, cgharibo at usa.netDate: Sun, 29 Dec 2002 14:42:44 -0500Subject: Re: Should we be using lighter weight oil?Chris:The multigrade oils have 2 ratings; The first number is the viscosity ofthe oil at normal temperatures (maybe room temp?) and is a goodindication of the rate of flow when the engine is cold. The secondnumber is the viscosity of the oil at working temperature, so a numberof 50 means that it has a viscosity of a 50 weight oil when at workingtemperature.This question was asked recently so there was a fairly technicaldiscussion which is probably captured in the archives, but I readnothing in that discussion that changed my thinking on the topic.The most wear on our engines occurs when we first start them up becausethe oil has drained down and the typical operating cushions of oilhaven't had the time to build up to operating pressures. When the oilpressure has built up there is a cushion of oil between all the mainbearings, connecting rod big end bearings, and camshaft bearings.Beyond that I think most of the engine is lubricated by spray and drip.So the most important thing for the longevity of our engines is to getoil where it is needed as quickly as possible. This dictates a low firstnumber since it is harder to pump the heavier weight oils when cold.After the engine is warmed up we want an oil that has enough body tomaintain that cushion and that dictates a high second number.In the good old days, before synthetics, a 10W30 was a good compromisebecause the standard oil couldn't have a much larger range. But todaywith synthetics, and with a 15W50 it sounds like you are using asynthetic, it is possible to have a much wider range than before. Mysuggestion would be to use a lower first number in the winter, perhaps a0, and a slightly higher first number in the summer. Do they recommend10 for the summer oil?But if you are using a synthetic why not use 0W40 (or 0W50 if available)year round? If 10W30 is recommended then you've got it covered :-)CheersGeorge> I have a '91 20V Coupe in the northeast with 20 degree winters and 90 degree> summers. I noticed Audi and just about every manufacturer is moving towards> 0W30, 5W30 type lighter weight oils. Should we be doing the same?>> I currently use Mobil 15W50 year-long, changed every May and November.>> What's your opinion on moving to lighter oils?>> Should I change oil to something lighter in the winters or all year long, such> as 10W30 or 5W30?>> Thanks,> Chris>

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