understanding oil weight

[JHHeiger]

Can someone clarify my thoughts about engine oil, please.

Is it true that higher numbered oil is thicker and has more viscocity?

Is it also true that the heavier the oil the more l pressure you'll develop, all things being equal?

And doesn't the 2nd number indicate that additives have been added to a 'straight' weight oil that contributes to a higher chemical stickiness?

thanks

[helowrench]

The first number iis cold viscosity, while the second is hot viscosity.

Mostly.

I did a science project with one of my kids, and can say that neither number is dead on what a straight weight oil is, but fairly close.

Yes, with a higher viscosity oil, you will make more pressure in theory, but that is only true in avw until one of the relif valves open, or it bypasses the oil cooler, and can actually make it run hotter.

[jhoefer]

Yes, higher number is higher viscosity.

Higher viscosity oil will produce higher oil pressure at the same oil temperature. As oil temperature goes up, oil viscosity goes down. As viscosity drops, then so does pressure.* Because the VW dual relief engine varies oil temperature using a pressure valve, what it's actually regulating is oil viscosity. That means when the engine is warmed up, a 10w oil should be running about the same oil pressure as 50w, but the 50w oil will show a higher oil temp. Of course things aren't always that simple and linear in practice. For example, low temps and thick oil can overwhelm the flow capabilities of the pressure relief valve and cause dangerously high pressures (blown seals/oil cooler).

Viscosity alteration isn't the only reason for additives. Many other reasons here: http://en.wikipedia.org/wiki/Oil_additive but in general, yes, for non-synthetics they use one base-stock oil that they add different things to to get the final multi-viscosity products.


*The oil pump is putting out a fixed volume of oil per second (at a given rpm). The volume per second of fluid flowing through a given diameter pipe is dependent on the flow velocity and the pressure drop across the pipe. The higher the fluid viscosity, the lower the flow velocity. Think of how fast things move when dumping out a cup of water vs a cup of molasses to see this is true. And because the volume per second entering the pipe is a fixed amount, the pressure drop across the pipe must increase to balance the equation. Since one end of our pipe (oil galley) is always open to air (the sump), the pressure at the start of the pipe must go up to have this larger pressure drop. Hence, your oil pressure increases with higher viscosity.

[tattooed_pariah]

I'm going to start this reply with "I'm kind of a special idiot and have never fully understood oil weights no matter how many times I read about them.."

With that being said, When i had my 1776cc built in 2002, I was told that the best thing to run for it would be flat 30, so that's what i've run for the last 12 years..

Lately I'm finding though that plain old flat 30 is damn near impossible to find locally, and when I do find it, it's at a specialty hot rod type shop that only sells break-in flat 30.. I found ONE pep boys near me that sells Pennzoil flat 30, but that's it..

What are you guys running? I'm at just about sea level (if that has any impact on it..), currently in Southern California so temps are pretty reasonable all year round, but in two months I'm moving to Virginia where it will be cold in the winter and hot in the summer.. what's the recommendations? The hot rod shop said he thought i'd be fine with a 5-30, but it's a V8 shop so who knows how much they know about flat4's..

[juki48]

if 30 works well then 5-30 or 10-30 should be fine. the real issue is make sure you have a good oil with good anti wear properties. I use brad penn 5-30. it is semi synthetic and has plenty of zddp for flat tappet cams, and isn't too expensive. there are zddp flat tappet cam oil additives that you can add to your favorite off the shelf oil as well. I just got 6 more quarts of the brad penn from summit racing to get my order to qualify for free shipping.

[tattooed_pariah]

Yeah, for my initial fill I used Lucas Break In additive with the pennzoil flat 30.. I haven't put many miles on it yet though (maybe 500 tops) so it's still in there.. I used to prefer Chevron flat 30 but like I said, I don't have many options anymore..

[Chip Birks]

I use that lucas additive with every oil change. They recommend half the bottle each time. I'm not too picky about oil brands when i use that stuff. Good old walmart super tech does me just fine.

[miniman82]

Where in Virginia are you moving? I am in Norfolk.

[tattooed_pariah]

Where in Virginia are you moving? I am in Norfolk.

Renting a house in Hampton, my ship is still being built at NPN.. I'll be there at the end of June with the wife and kid