Oil Weights and such

You on the right track, but not quite.

The 5W/10W/15W/etc means that the oil is within a certain viscosity range at 0 degrees C (32 deg F) as defined by the Society of Automotive Engineers (SAE). The second number means that the oil is within a certain viscosity range at 100 deg C (212 deg F). The oil will vary in temperature - lower near an oil cooler - hotter on the block. It can vary in between. So to answer your question - it would be when it's near full operating temperature.

Theoretically a 10W-30 will give marginally better protection than an equivalent 5W-30 (high thin film strength) - all things being equal. Then you run into synthetics and the amount of anti-wear additives.

It can take anywhere from a few miles to 20 miles for the oil to become warmed up. This is relative because you can drive all day in cold weather and the oil will not get as hot as it does after a few miles in hot weather. In a perfect word the oil thickness would not change with temperature, but in fact it changes a lot. When you select an oil viscosity you have to decide if you want a low viscosity oil to get the best possible gas mileage when starting up or a higher viscosity oil to prevent wear when the oil is hot and thinned out. Keep in mind that 20W50 oil is not as thick in hot weather as 5W20 is in very cold weather. Synthetic oil is usually not worth the cost except at temperature extremes e.g. in a turbocharger in hot weather or in any engine when the temperature is lower than -20 degF.

JP wrote:

||I'm a newbie DIYer and I was pondering on the issue of Motor Oil weights. ||During my short 19 years on this great earth, I have come to understand that ||the first number always indicates the starting consistency of motor oil. ||i.e. 5W is more fluid than 10W and that than 20W. The second number ||indicates the oil's consistency after start-up. But just how long after ||start-up? Do both 5W30 and 10W30 give essentially the same protection once ||operating temp. is reached? One last nagging question; What kind of ||application would deserve synthetic and what not? Thanks in advance, all ||input is welcome || ||JP ||Silverbullet || ||

Rex in Fort Worth

Years ago a mechanic told me that he was at an oil presentation by an oil company engineer, who said that once the oil gets dirty the dirt interferes with the molecules which curl and uncurl to change the viscosity, and the oil ends up being pretty close to a single weight oil.

I don't know if this is true but it's worth checking into if you care that much about the viscosity. I don't, in my book the oil level and frequency of changing are much more important than the difference between 5W-30 and 10W-30.

Ted

It may not be AS TRUE as it once was. A lot of the so-called conventional oils these days are Type II (or hydrocracked petroleum base). I believe the process produces a base oil with less variation in molecular size. The end result is supposedly a base oil with a higher inherent viscosity index - i.e. it thins out less as it approaches operating temps. This base oil should require less VI improver to make it (let's say) a 5W-30 oil. True PAO/ester base oils have an inherently high viscosity index, and the blend of the two supposedly improves the VI. I've heard on occasion that Mobil

The problem with older cars is the seal materials and untested oils. PAO supposedly has a tendency to harden and shrink seals by causing the plasticizers to leach out of the rubber. Esters counteract this shrinking. However - I believe that seal materials have been designed with synthetics in mind, and the oils have additives so that it shouldn't be a problem. This shouldn't be much of a problem with newer engines.

G'day, with all this different oils on the market with different additives reminds me what my grandfather says about it.. He still uses straight SAE 30 in all this vehicles on the farm.. says that the thicker oil will stick and not run off like 5w30, so it won't matter if the oil takes a little longer to flow cause there is already a good film on the cylinder wall, as where the 5w30 has to flow quickly to recoat the walls because it's to thin to stick.. Not too sure about his reasoning.. but it did sound interesting..

However - straight weight oil isn't going to be very good in cold weather. It's likely to not be or contain less hydrocracked base oil. Fuel economy will go down. I'm surprised it's still sold.

Actually - synthetic oils with an ester base form a good coating on the cylinder walls after shutdown. They stick due to static bonding. So do your typical antiwear additives in any modern motor oil.

Interesting! Sometime I mix oil viscosity (same brand just diff. vis cosity). For example, 10W30 with 20W50 to improve oil viscosity after engine warm up (at 100 oC). Is it okay?

'Pennsylvania

requirement.

condensation

interesting..

Thank you all for your input so far

If that's what you're looking for, then why not try a 15W-40 oil? They're generally meant for diesel engines, but should be fine in gasoline engines. My new car's owner's manual says that 20W-40 can be used in extreme heat of for towing (15W-40 is close enough - especially since I've never seen a 20W-40 oil on a store shelf).

If you mix 10W-30 with 20W-50, I'd say that the result will likely be somewhere in between, but still unpredictable. I've mixed oil weights myself, so I don't think it's a disaster waiting to happen or anything.