Confused about single and multi viscosity

[noi8u]

Okay, I've been spending some OCD time online and on the phone and I can't quite get a straight answer to my question about how a Royal Purple 0w40 and a single SAE 40 functions at a given temperature. So far I have been told that when the outdoor temperature is over 60 degrees and the 0w40 is hot it is a 40 and that the SAE 40 is a 40 when the engine is cold or hot. I have also been told that engine wear would be greater on start up with the SAE 40. Now if the 0w40 is a 40 when the engine is hot, and you shut it off then start it again are you not starting the engine with a 40 weight oil just the same as the SAE 40 when it is cold or hot?

 

[Quattro Pete]

Originally Posted By: noi8u
Now if the 0w40 is a 40 when the engine is hot, and you shut it off then start it again are you not starting the engine with a 40 weight oil just the same as the SAE 40 when it is cold or hot?

When it's cold, a SAE40 oil is much thicker than a 0w-40 oil. Since most wear occurs at cold startup, you don't really want an oil that's as thick as molasses.

As the oils heat up, they become thinner and thinner. By the time they reach operating temp (say 100C), they're both just as thin.

 

[noi8u]

I understand all that, but what I'm getting at is when 0w40 oil is hot whats the difference on start up from the SAE 40 when it's hot? If the 0w40 is a 40 when a hot then when wouldn't it be like molasses too?

 

[Quattro Pete]

Originally Posted By: noi8u
I understand all that, but what I'm getting at is when 0w40 oil is hot whats the difference on start up from the SAE 40 when it's hot?

There is no difference. The oil is the thinnest when it's hot, and at that point both are just as thin.

What I was trying to explain is that it's not the hot startup that's the problem. It's the cold start up, expecially on a cold winter morning, that puts a strain on the engine.

Quote:

If the 0w40 is a 40 when a hot then when wouldn't it be like molasses too?

Again, when these two oils are hot, they are both just as thin. This is as thin as they'll ever get under operating temperature.

But when they cool, they become thicker and thicker. The only difference is that a 0w-40 will not thicken as much as a SAE40. When the temps drop below freezing, the SAE40 may become the consistency of tar, while the 0w-40 will still be relatively free-flowing.

 

[Pablo]

https://bobistheoilguy.com/forums/posts/1593572/

 

[maximus]

How come piston aircraft engines ususally use single weight oil, even SAE 50..in the winter?

 

[maximus]

Originally Posted By: maximus
How come piston aircraft engines ususally use single weight oil, even SAE 50..in the winter?


Nevermind. I realize now it's up to the individual operator. I'm just used to seeing rows of Aeroshell W100 on the shelf at flight schools I've been around. It's easier to stock one type of oil. But [censored], in college, we used to pour straight SAE 50 into our Cessnas in JANUARY! That stuff is literally like molasses in cold weather.

 

[noi8u]

See that's my point. Say I have a turbocharged engine that consumes oil why not just add SAE 40 in the summer when the temp is 60-100 degrees? Or even mix it just to get some sealing. I've poured VR1 50 and it's not that thick. Hey it was only $4 and I'm getting sick of adding oil.

 

[Quattro Pete]

Are we talking about that 2.0T engine of yours? I seriously doubt this amount of consumption will stop with just using SAE40 in it. You need to figure out what's going on with your car as it looks like you have a whole set of issues there.

 

[noi8u]

Yeah I got a great deal on a demo and audi service just placates me. They really don't have good service where I am. Must be why everyone has BMWs with 10w60. Hey didn't they start out with aircraft engines lol.

 

[mechtech2]

The 0-40 would be much thinner at 0 def F than the straight 40.
And like you surmised, it would be thinner at the intermediate temperatures until it reached 212 F.

Aircraft piston engines are operated at nearly wide open throttle.
Lots of heat and stress produced in the engine.
The extra stability of the straight weight oil gives an advantage. At least in the past. I'm not sure about today's oils.