Synthetic Oil weight ?

[amalventano at sc.rr.com (Allyn)]

> (Mike)
> 1 5w 50 (red cap) in the warmer parts.

> (Larry)
>  You want to vary the FIRST number in the oil's viscosity rating according
> to temps, not the second.

he means 15w 50 (they dont make 5w 50).

also, the first number is the viscosity at 0F. the second number shows the 
equivalent cold viscosity that a hypothetical oil would have to have to 
match the oil in questions hot viscosity (at 100C).

so for example, applied to the 15w 50, its cold (0F) viscosity would be 15, 
and its hot (100C) viscosity would be no less than that of 50 weight (@0F) 
that was heated to 100C. this is possible by adding polymers to the oil. 
these polymers effectively thicken the oil as it heats up, which attempts to 
offset the natural effect of the oil thinning out as its heated.

so, since most of an engines running time is when its hot (100C), it can be 
argued that the second number plays a more important role in oil selection. 
in older engines, where there are larger clearances between parts, thicker 
oil can help minimize damage to these parts as they attempt to impact 
eachother.

the 'normal' disadvantages to using thicker oils are increased slidge build 
up (not an issue with synthetics), and harder starting (not as much of an 
issue with mobil 1's cold 15w value, 5w lighter than the typical 20w 50 oils 
out there). there is also slightly increased fuel consumption, as its 
physically harder to spin the engine parts in the thicker oil (if you're 
worried about the engine life, you wont be bothered by the 1-2 mpg 
difference).

also, if you dont use synthetics, its important to note that lighter 
conventional oils add more polymers to get their relatively high hot 
viscosity values. the polymers are the part of the oil that breaks down over 
time ("thermal breakdown"), so its wise to select oils with low 
added-polymer counts. in general, the thicker conventional oils use less 
polymer additives, which leaves you more 'oil in your oil', resulting in 
higher overall protection. as long as you can deal with the disadvantages 
(listed in the previous paragraph), its better to go with the thickest 
stuff.

hth
Al

Allyn Malventano, ETC(SS), USN
87 Rieger Scirocco GTO 2.0 16v (mobil 1 - 15w 50)
90 Passat GL 16V (mobil 1 - 15w 50)
86 Kamei Twin 16V Turbo Scirocco GTX (will have - mobil 1 - 15w 50)
86.5 Occo 16v Trailer (umm, it will have new quarts of 15w 50 stored in it 
:)