Need help with OIL numbers...

[72mustang]

Ok... im dumb when it comes to numbers in oil ratings. The lower the number, the thinner the oil????

The first number is the thickness of oil when its cold, and the second number after the W is the thickness of the oil when its hot??? Or do i have these backwards?

Reason im asking is... a 5w50 oil is like molasses when pouring out of container cold... so how could it go from thick ( 5 ) to an even thicker ( 50 ) if this is how the numbers are rated?

Please educated me....

[BenjiBoy650]

You are correct, lower number = thinner oil and the second number is hot, the first number is when cold.

I asked the same question as you a few months ago and what I concluded is that the oil actually does get thinner as it warms up. The disparity in the numbers is due to them being rated on a different scale. The numbers are comparable on the same scale (5W-30 is thinner than 5W-40 when hot) but cannot be compared to each other (cold 20W-50 is not the same viscosity as hot 5W-20).

[72mustang]

Thanks.

Help me out just abit more....

I own a motorbike. Its rated for 10w40 oil.

I put in 5w50 recommened by a local rider with same bike.

I find when i first use the bike.. ie.. its not fully warmed up... it shifts gears perfect and smooth. After 10 minuites or so of being warmed up.. i find the gears abit stiff and clunky. So im thinking it needs thinner oil? So the 5 is perect for starting... so im thinking maybe changing it to 5w30 or 5x20. What do you think?

And finally.... how come the 5w50 oil is ALOT thicker cold than 10w30 or 40 cold? I though the 5 meant it was thinner than 10 ?

[BenjiBoy650]

Are you referring to the tranny oil or the engine oil in your bike? If it's the tranny oil, then it makes sense. When the oil gets warm, it's thicker than it should be and so it might be harder for the tranny to cut through it. The oil should be thinner when it's warm, so you may think that it would shift better after it's warmed up, but remember that parts expand when they warm up too so depending on how it was designed, it may go either way with the thinner/thicker oil. Play it by ear, use what has been recommended but also realize what works and what doesn't from your own experience. If it's engine oil...then I don't know. The extent of my bike knowledge is that it has 2 wheels and moves :thumbsup:

http://www.bobistheoilguy.com/visc.html - viscosity rating info
https://www-auth.cs.wisc.edu/lists/vfr/2002-September/msg01264.shtml - good description of "thick" and "thin" at different temperatures
http://en.wikipedia.org/wiki/Motor_oil

The API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. The first number associated with the W is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as cold as a single-grade 10 weight oil can be pumped. "5W" can be pumped at a lower temperature than "10W". "0W" can be pumped at a lower temperature than "5W", and thins less at temperatures above 99°C (210°F). The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle.

^^ I read this before but forgot about it...dunno how to answer your 5W vs. 10W question except that maybe it is just a matter of perception? :dunno: