5w-40 5w50
 

Any reason why a person would choose a 50 over a 40?

:cheers:

Better film strength under higher temps; 15W-50 used to be standard fare in the aircooled 911's......

I'm doing more oil testing in our track Boxster and plan on trying out Syntec 5w40 as well as 5w50 to see how it compares to previous oils, which have included Mobil 1 0w40, 10w40 4T Racing, Brad Penn 10w40, and Motul 8100 5w40. Next fill is Joe Gibbs Racing 10w40.

Charles, isn't there a general rule that it is always best to have the two numbers closest together? For example a 5W40 vs. a 0W40 or a 15W50 vs. a 5W50. With the obvious understanding that the specific viscosity needs to meet the manufacturers requirements.

In general, this rule used to hold true. With synthetic oils, this is supposedly a bit less of a concern, especially for true class 4 and 5 syns.

Not at all. These are just grades determined by a methodology developed by the SAE. They do not represent actual weights of oil, but are in fact arbitrary numbers assigned by the SAE.

The dual grade numbers indicate that it's a multi-viscosity oil. That is, it's viscous properties (resistance to flow) vary with temperature due to the addition of long chain polymers. These polymers curl up in cold temps making the oil less viscous ('thinner'). When the oil temp rises, these polymers uncurl making the oil more viscous ('thicker'). Viscosity is a measure of how a fluid resists flowing, less viscous and it flows easier, as the viscosity increases, it is more difficult to flow - water vs honey are good examples. The 'W' stands for Winter, not weight as commonly mistaken. It indicates that at 0°F the oil will flow at the same rate as an oil the weight of the number value before the W, in a 5W40 oil, this would equate to a 5 weight oil.

The benchmark temperatures are 0°F, and 100°F. So, at 0°F (or below), a 5W40 oil will flow like a 5 weight oil, at 100°F, the oil will flow no less than a 40 weight oil. An oil with too low a viscosity can shear and lose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.

Back in the day, people used to use different weight oils with different seasons (temp ranges). Multi-vis oils eliminate the need for this.

But understand that the actual base oil may not be either 5 weight or 40 weight, and usually isn't - it's usually 10 weight. It's just that the combination of polymer additives and the base oil give it the flow characteristics of the oil weights in the assigned grade - 5/40. Now, as the temp increases beyond 100°F, the oil continues to thin out. A higher graded oil, or bigger number value, will thin less than one with a smaller value, but not necessarily with any lineality.

:cheers:

Is your name "Charles" too? I thought it was.......oh, never mind. Please, in the future, don't answer a question of mine that is specifically address to another Forum member.

Quote: "Is your name "Charles" too? I thought it was.......oh, never mind. Please, in the future, don't answer a question of mine that is specifically address to another Forum member."

Posted by Flavor 987S.

There's another thread out there that's calling your name. As I recall, it opens with "I'm a douchebag!" Right down your alley...

Threads---I always thought, anyway----are for all to read and, if so inclined, to respond to. That's kinda the whole idea. If you really don't want others (many of whom are pretty knowledgeable) to respond to a post, then PM the question you have to the person you do want to hear from.

Seriously. Charles may or may not even respond to the question. Lil was trying to help answer the question for you, and even offered a friendly beer chug at the end of his response. Sheesh. When you post a question on a public board, anyone can read it and anyone may respond. If you only want one person to read and respond, PM them, as Frodo says.

Play nice please! :)

Since you're new here I would suggest sending a pvt msg if you don't want people taking the time to help answer your questions. We all try and contribute, so if you don't like it just send a pvt msg.

:cheers: