Oil thickness and grades

[mattysshop]

now i know this may seem like a silly question, but i have always gone by what the oil bottle said, and i was fine.. but recently some of my mates have brought up very good point, and now i'm just confused!!

take for example..

a 15w 50 and a 14w 40 oil..

15 meaning the cold start viscosity, and 50 meaning the operating temp viscosotiy, from what I have read, and people have said.. 15 is thicker than 50..as the 15 is used on a cold crank, thicker oil will create more oil pressure there for reaching the top of the motor quicker etc..

so why the hell would a 10w 40 (same SAE grades) be insanely thin and blow through rings and create massive valve train noise as if the oil was just tooooo thin?? If going by what the book says… the lower the number the thicker the viscosity right??

Just incase people were wondering Ambeint temp would be around 25-27deg C so both oils would be fine working in an average engine..both oils are same brand and both synthetic blend

Am I missing something here?? Physics proves that pretty much all liquids thin out as the temp increases...especially oils..

i'm just confused now with this over load on info...

Matty

[monkeymajik]

The smaller the number the thinner it is.

[mattysshop]

so oil defies physics and gets thicker when hot?? why is it that mechanics test drive a car befor a service, i always do it to as the oil is hot, and drains a hell of alot faster cos it's thin..

[mattysshop]

this is what i don't get..because that used to be what i thought.. then i realised it's not the case.. oil gets thinner when it's hot.. service a hot car takes you 5 mins.. service a cold car you can at least double that...

[crazy86]

Ok, let me start that i dont know the answer, and im not really sure what Im talking about, but i seem to remember reading that the second number is not "thickness", in the same manner at the first number, but it is more a number to signify the rate of change of the oils viscosity once heated up to a certain temp.

therefore a 20w 40 oil will change viscosity less than a 20w 50.

and a 15w 40 oil may have a similar "hot" viscosity to 20w 50

Then again, i could be completely wrong and leading everyone up the garden path

[mattysshop]

on the back of most Penzoil bottles it says cold start viscosoty and operating temp viscosity, but yeh that would make sense a bit...

i know for a fact that a smoky engine running on 10w would benifit greatly running on a 20w.. so that to me means that the higher the first number the thicker the oil?

[monkeymajik]

Maybe i've read it all wrong too

I think crazy86 is right though, because the higher the second number the more protection you have in hotter environments.

The smaller the first number though the thinner it is. I am certain of that.

[crazy86]

yep ^^^matty

[mattysshop]

RIGHT!! good work fellas! the milkybars are on me! now that i've got that sorted.. i'm getting me some 25w50

[Monk]

he he..

he..

I still dont get it

I know nothign about oils but from what i do know (which is nothing) the lower the number the thinner the oil.

5w (cold) means thinner, little protection.. but more performance
20w (cold) means thicker, good protections .. little performance gain?

thats what i was always told. i could be wrong but?