Visco City

[padgett]

Having seen a lot of confused posts on the subject, thought I would take a moment to explain what the difference between 5W-30 and 20W-50 really is.<P>The standard is set out in SAE J300(my version is "d" but is from the 1980 handbook - YMMV).<P>The first number is a description of how thick the oil gets when cold, at -18C to be precise hence the W-Winter designation.<P>5W gets no thicker than 1,250 centipoises, 20W 10,000 centipoises so 20W is about 10x thicker when cold than 5w. 10W is 2,500 so only gets about 2x as thick as 5W. "15W may be used to identify SAE 20W oils that get no thicker than 5000 cp so 15x is 2x 10W which is 2x 5W. Log scale.<P>Now, just to confuse things, the second number is the hot viscosity (at 100C) and is measured in CentiStokes (centipoise = centistokes X specific gravity) so to make sense you need to know the specific gravity of motor oil. .88 is a pretty good starting point. <P>SAE 20 is 5.6 to 9.3 CS<BR>SAE 30 is 9.3 to 12.5<BR>SAE 40 is 12.5 to 16.3<BR>SAE 50 is 16.3 to 21.9<P>now this goes much higher (140 wt axle grease for instance) but this is the typical range for motor oil. <P>The point is that 5W (1250 CP ~ 1420 CS at -18C is <span style="font-style: italic">far</span> thicker than even 50 weight when at 100C (21.9 max). Consequently, I doubt that 20W-50 is going to hurt anything so long as the weather is well above freezing. e.g. a car kept in a heated garage or in central Florida.<P>Now 20W-50 is not considered "energy saving" because of the thickness and CAFE is probably the reason for the 5W-30 spec rather than longetivity.<P>As mentioned, the ambient here rarely goes below 32F and my cars are garaged so I use 10W-30 year round.<P>Final note: when I was in school, multigrade oils were achieved by addition of long chain polymer additives to "fortify" the oil. You started with about a 20 weight oil and blended it to reach 10W-30 characteristics - not so thick as straight weight at -18C and thicker at 100C. <P>Over time these long chain polymers break down and instead of a 10W-30, it reverts to something more like a straight 20 weight. This is why my rule is to watch for a slight drop in hot pressure and then change the oil. <P>This is why the idle pressure just after changing the oil is usually higher than before if the oil is 2-3k old, new oil has not started to break down yet.<P>Synthetics are different, modern ones are very stable and do not break down. Consequently they can start out thinner than conventional oils (isn't Mobil 1 a 5W-15 ?). However they can become contaminated particularly in an older engine. Blowby is the most common cause even in a good engine but a slightly leacking PCV valve can do the same thing.<P>Have used both but personally prefer to just change oil and filter often.<P>BTW I used AmzOil in my Sunbird until it got to the Chevvy range of 600 mpquart. Did find quite a bit of gunk in there when I swapped intake manifolds after a few years.

[sahein]

The Mobil-1 that I use is 5w-30 but there is a recent addition to the Mobil-1 line of 0w-30.<P>Back in the big V8 days I used to run 20w-50 and change it every 5k miles. Some manufacturers even recomended intervals as large as 6.5k miles. Now that I am wiser, I use 5w-30 and change it every 3k miles. <P>The largest engine wear occurs from contamination in the oil. The wear to contamination plot is an inverted bell curve. Once the contamination reaches a certain point, the wear and contamination go hand in hand feeding each other to accelerate distruction.<P>Piston ring loads are increased as the pistons move up and down in the cylinders scraping away heavier viscocity oils. At higher rpms the newer modern type of lower pressure rings may not seal properly. When the rings don't seal properly more oil is introduced higher into the upper rings and combustion chamber. This leads to accelerated carbon build up.