Your temp assumptions are correct, if the oil was built in the middle of the grade, or the high end.

What if you bought a 10w30 and the oil, out of the can, was on the low end for a 30? At temp it is out of grade.

IIRC, to pass the API grade, and oil only has to pass the test 5 times out of 10, again, IIRC. They don't have to pass 100% of the tests.

That's why testing oil is expensive. You have to monitor oil temps very close, among other things.

IMHO, a good oil is built on the high end of the grade. That's one reason why it's difficult to compare a 40 of one brand to a 40 of another, without knowing EXACTLY where the oil tested in the grade. One oil could be on the high end (almost a 50) and the other on the low end (almost a 30) and you end up spending a bunch of time trying to figure out why the high dollar oil is down on power (assuming the expensive oil is on the high side of the grade, which most of the ones I know of are, and the cheaper oils are usually on the low side of grade) compared to a cheap oil. In that case, you would test a high grade 30 (almost a 40) against the cheaper 40 (almost a 30) and th results would be more accurate.

Why would the cheap oil be towards the low side of the grade number? 20,30,40 all costs the same. Less film strength, additive package, and base stock you bet, but viscosity?
Doug

It has more to do with fuel economy and incompetent regulation, like CAFE standards.

And all oils don't cost the same to manufacture.

Oil temp will easily exceed water temp if you are using block filler. With 1/2 fill, my oil temp sees 212 while my water temp is at 180. - 10W30 Royal Purple full synthetic race oil. Per Royal Purple, most run too low oil temps - claims their synthetic will easily handle 250+ temps with no ill effects. Think they were mostly referring to expunging water from the crankcase. At idle start up I am looking at 70 psi. At idle and 212 oil temp I am looking at about 25 psi.( makes me a little nervous) Yes, it has been my experience that synthetics are more prone to leaks. I switch to 10W40 in real hot weather.

I wouldn't and don't use block fillers in any of the engine I build due to the oil temps and cylinder wall heat and expansion differences I'm old and I don't like a lot of the changes in a lot of the things we have to deal today
I regularly saw 15 to 20 lbs oil pressure at 850 RPM idling in gear with my duster with 180+F oil temps with that 5w20WT oil, 30+lbs at 1200 RPM
As far as oil temps my air cooled airplane motor had a oil thermostat to make the oil get up to 190 F before opening the lines to the oil cooler so it would boil the water out of the motor from condensation I used 15W40Wt in it.
I was told in commercial flight training that petroleum based oils would start to deteriorate and destruct the oil molecule at or above 325F, synthetics where good to 500+F
ATF has added dyes that will start to change colors from red at 275F and be black at 325F

ANY oil pressure at idle hot is fine. Our 500" inline 6 cyl CNG buses barely register @ idle hot and have NO issues, dragging around 33K pound transits running 21psi boost. The big diesels are the same lot's of boost and hardly any hot idle oil pressure- 10 psi/1000 rpm just works, more than that you are throwing away power and overheating your oil by pumping it past the bypass back into the crankcase.

If you don't run any spring pressure. The higher the spring pressure, the harder the low oil pressure at idle is on cam bearings, pushrod cups, adjusters and rockers and shafts. Anything over 120 on the seat and you need more idle oil pressure.


Roller cam bearings have more than one person.

As far as viscosity at specific temperature goes, that can be calculated or graphed. You just need the viscosity at 40 and 100 C which almost all manufacturers provide on-line if you hunt. This is also a way to compare specific oils, say a brand X PAO synthetic vs. its conventional, vs a Brand B semisynthetic.
Viscosity Graphing page
Operational Viscosity

As far as how far they fall off in use, that's a good point and will require sending a sample of used oil to a testing company.

PS. Brad - Thanks for posting that!

I brought this up a while ago when I freeze framed the crew strapping in Greg Anderson and Erica Enders at a race and saw the oil was around 20 lbs at idle and figured we're good. The 10 lbs per 1000 seems safe enuff............

Like I posted earlier, you have to remember PS has extra cam bearings, and they are almost always roller bearings. That alone allows for much less pressure, and way less volume of oil at the cam.

There isn't much oil in those engines that isn't accounted for.

Smaller bearings, rigid blocks and cranks allow smaller clearance. Cold blocks and coolant allow thinner oils. Thinner oils equals less pressure.

Not a good idea for the average guy to follow the PS lead on this.

David Reher has published this and it can be found on the RMRE web site.

I understand those are purpose built race cars which makes sense and I have been running 5w30 wt oils for years now and see temps from the teens to 114+ and my bearings looked real good after my usual 5+ year teardowns/freshen ups. When I see guys recommend 20w50 because of wider(race)bearing clearances I cringe unless your cars sits in the sun at 100+ degrees before start ups............

Not that uncommon to race in 100* temps in the Phoenix / Tucson area. Pulled in to the track in Tucson a few years ago and it was 105 - in the evening!! Yes, 10 lbs per 1000 rpm is generally accepted as the standard. What is the general rule on viscosity change when switching from petroleum to synthetic? - all else being equal.

Depends on whose oil you start with, and whose oil you change to.

I'd never change oil without talking to the manufacturer of the oil.

You can buy a 40 that is almost a 50. What if you are running a 40 that barely makes a 40 and then you switch to an oil that is a 40 but is almost a 50?

There need to be more time spent learning about oil.

Testing oil is very costly. I have seen testing done in a sloppy manner, where a great oil made less HP than a cheap oil. It was piss poor testing.

That was my point about 100+ heat allowing for the thicker oils and I just changed mine and went w/5 qts. of Valvoline Syn. and 2 qts. of regular dino Valvoline both in 5w30 and a K&N 3001 filter.........Even in the summer I street drive w/thinner oils and at the very hottest here in the high desert where 110+ is common and hotter, I still have 25+ oil pressure at idle and it quickly jumps when revved...........

On the Max, it's best to use the Fel-Pro 1218 and then coat them all around with a shellac type of sealant before installing, Jerry Stein taught me that trick.

The spec sheets available for pretty much every oil I'll ever use contain the viscosity #s that are used to find a "heavy" 30 or a "light" 40. That's why I posted those charts previously to make it easier for people to see things like this.

Oh, I haven't seen any pattern when checking the #s that indicates the "better" oils come in at the high end of their advertised weight. The brand doesn't matter; they can be all over the viscosity range for a given weight.

There isn't one. Viscosity is viscosity; synthetic oils advantages start with the ability to operate under higher temperatures before they break down. The additive package for one synthetic could be weaker than a comparable petroleum, so there's no guarantee it's a "better" oil under normal operating temperatures.

Exactly. The manufacturers have a wide field to work with as far as viscosity is concerned.

And your correct, most manufacturers post their oil specs on a web page. I'd rather call them

I keep reading about stiffer blocks and cranks. Well, maybe the blocks if going from grey cast (20 million PSI) to nodular or Compacted Graphite (25 to 30 million PSI).

For the cranks, however, if we start out with a standard stock forged crank and change material to 4340, it is maybe 3% stiffer.
When we change the journal size to smaller diameter the crank loses stiffness, it is a natural fact. There is no material that is stiffer than steel that a crank can be made from, except molybdenum, and I don't remember ever seeing molybdenum in its pure form, ready to be cut into a crank. Plus, moly is about 30% more dense than steel.

R.

i took some really cool classes in college, and if i remember correctly, the winter number is not a viscosity, but a behavior. it BEHAVES like a 10 weight oil. it equals the lubricating qualities of a 10 weight at the specified temperatures. it was exactly one million years ago tomorrow, so i could be wrong.