[deepblueQ]

I would stay far away from anything xW30. What the hot climate basically means is that the second number is far more important than the first.

The first number is the viscosity at 104*F, the second is viscosity at 212*F. In that kind of climate, you'll be very near 104*F right when you start up, and you'll spend most of the time closer to 212*F.

According to the chart in post #2, MB doesn't think xW30 will reliably keep bearing surfaces in the engine from touching each other in those temps.

If you do use 5W30, I think you should use a much narrower rev range... use manual mode, and keep an extra 500 RPM away from lugging and 1000 RPM away from redline...

 

[deepblueQ]

And how do you explain Ford & Honda using 5W20...?

Their engines are designed differently.

Bearing tech [epi-eng.com]

BOC = Viscosity x RPM x Diameter x K / Unit Load

To avoid bearing wear, we're trying to keep BOC above a certain point (about 35 in that apparently unitless explanation). When you go below that BOC, that's lugging. I realize that that's not how lugging is sometimes defined, but I think it's by far the most useful definition.

Take the case of a smart cruising at 35 MPH in 5th, a case that a lot of us seem to think of as lugging. The TCU was programmed to do that, so it's obviously not a problem. The MB engineers thought that those conditions gave a reasonable BOC when using an oil that's OK by the chart in post #2. If you use a lower viscosity, that lowers BOC... possibly below what's acceptable.

To compensate for it, you have to either have bigger bearings (diameter and unit load) or use a higher RPM.

Engines that spec 5W20 have to either use bigger bearings or higher RPMs to be able to do so.

 

[deepblueQ]

It's probably worth noting that the oil charts outside of the US do, in fact, list 0W-30 and 5W-30 as being appropriate for temperatures above 86 degrees.

Evilution has a "rest of world" chart for reference: Evilution - Smart Car Encyclopaedia

The engines for the rest of the world are also built differently. It could even be something as simple as the gearbox being programmed with a bit higher shift points.

Part of the reason I just had the oil changed and was not concerned about using Mobil 1 5W-30, the other being if it's good enough for an LS6 engine it's good enough for the engine in the smart. YMMV, of course.

An LS6 may be built with bigger bearings than a smart, and/or may not be revved the same way.

 

[deepblueQ]

>They are? How so? Specific differences, please.

>It's a 350 ci V8 so the bearings are certainly larger; same 6.5K redline as the smart, with six times the horsepower and accompanying loads. I think the smart will handle Mobil 1 5W-30 just fine.

At least older US-spec smarts were 10:1 compression ratio, while the N/A smarts for the rest of the world were 11.4:1. It remains unclear what compression ratio new US-spec smarts are, but the old ones definitely weren't ROW-spec.

The LS6 has bigger bearings, but it also is designed to operate at lower RPM, has a bigger bore and I assume a longer stroke (leading to higher loads)... there are too many variables for us to say how things work for them as compared to a smart.

Do you have any actual proof of this? I'd be extremely surprised if Mitsubishi were supplying special "US-spec" engines to smart, and even more surprised if Mercedes-Benz used something such as minor shift point variations (which we also have no proof of) as a basis for the oil chart in the US manual.

The more likely scenario is that the chart was intended to more positively portray 0W-40 as opposed to the more commonly-available 0W-30 or 5W-30 for use in all US climates (of which Mobil 1's 0W-40 is the most readily available "MB-approved" variant here in the US).

See above. New smarts may have been standardized to ROW-spec, but the full oil viscosity chart was released before that happened even if it did.

 

[deepblueQ]

Don't worry, I'll let everyone know if the smart blows up....

You're in Ohio, so if you do go above 86*F it probably won't be by more than the margins they spec'ed into the shift points. Now, if you were to go for a 4th gear highway run on a 95*F day, then get back into town and back to auto mode, that would probably be very bad...

I use 0W30 myself, I'm just careful with the revs on hotter days.

Now I am getting even more confused (between jwights chart & evilutions chart) LoL! Because if I can use a 5w-30 or even a 10w-40 then I can get those for a much lower price then do the change @ every 5,000 and be fine with that...BUT now what about semi-synthetics? I ask this simply for the reason that my local shop does a semi-synthetic 5w-30 oil change for $9.99! Now, I am not trying to be "cheap" (as I don't mind paying for confidence,safety and security) but since I will be doing a minimum of 500+ miles a week my oil change schedule will be a lot more often than most and being frugal/economical in these times is not the same as being "cheap".

10W40 will be just as good as 0W40 as far as engine wear goes (at least in your climate), but if you use the 10W then your fuel economy will really suffer until the engine gets warmed up. Much more of an issue if you make lots of short trips, as opposed to fewer longer ones.

I'd still stay away from 5W30, but it's your car.

I'd also stay away from semi-synth, but I'm not nearly as opinionated on that as viscosity. Chances are you'd never know the difference for using it... it'd just be more wear of the gradual sort.

Some of the earlier documentation stated that they were 10.0:1, but I have yet to see anyone present actual evidence (different part numbers across the model years for pistons, rods, etc.) to indicate that this discrepancy was anything other than an erroneous figure in the manual.

Even if the earlier US-spec cars were in fact delivered with a lower compression ratio, the change would not necessitate a different oil specification from that of the ROW cars running a higher compression ratio. And as the higher compression ratio began being listed in the literature at least as early as 2011, it would certainly offer ample time to "update" the oil chart accordingly to ROW-specs if that were indeed the reason for the original modification to the 30 weight values.

Larry's screaming cow got to 23 lbs of boost on stock internals before it blew up, and when it did it wasn't because of knock, burned valves, preignition, or anything like that... it was just that a compression ring wasn't sized right for those temperatures. As far as I've heard, there aren't many engines out there with normal 9.x:1 compression ratios that can take that kind of boost on stock internals. An 11.4:1 engine that could handle abuse like that would be quite a feat indeed.

To put things in perspective: 22 lbs = 1.5 bar, so 2.5 atm absolute... 2.5 * 11.4 = 28.5... 28.5 / 9.5 = 3 atm absolute, so 22 lbs @ 11.4:1 is roughly equivalent to 30 lbs @ 9.5:1, except it's even crazier than that since a bigger percentage can be intercooled on the lower compression engine.

Do you know of any originally N/A engines that can take 35 psi boost on stock internals?

The chart on Evilution's site is the one smart has provided outside of the US for a number of years.

If you have difficulty accepting that Mitsubishi somehow re-worked the 3B21 exclusively for the US market in such a way as to require different lubrication than the engine it provides to smart for the rest of the world, then the Evilution chart is fine.

You might also be interested in learning that it had previously been determined that Mitsubishi delivers the engines to smart filled with a non-synthetic oil (most likely 5W-30).

Mitsu most likely doesn't do the ECU/TCU programming. The ECU is Bosch, the TCU is most likely Getrag. There's a lot of room for difference there... did that source mention if MB changed the oil before sending them out?

 

[deepblueQ]

The fuel economy when "cold" per se shouldn't be that much of an issue since it is always hot down here, thus between the intake air temp being hotter and the ambeint temp of the block being warmer (because the car sits in the sun all the time) I think 10w-40 would be the greatest compromise and effectiveness in my sitaution as it will get to operating temperature quickly. So I believe I can get a less expensive (but still quality synthetic oil) and do them myself easily/quickly at a 5k interval for my peace of mind!

10W40 will still be around 2/3rds thicker when you first start it up than 0W40, even in your climate. 5W40 would be much better.

 

[deepblueQ]

I've got some Mobile 1 ,0W-20 around. Can I run this safely in moderate weather at freeway speeds ? A2Jack

Moderate weather means W30 would be acceptable by the MB spec, so let's use that as the baseline.

W30 = 10-13 cSt when warmed up, W20 = 6-9 cSt when warmed up. 7 / 12 = 0.58, so when using 0W20 you should only assume you've got 60% or so of the BOC you would with xW30.

Use manual mode and keep it revved 60% higher than you normally would when lugging is the issue (I'd say use 5th strictly for maintaining highway speeds, and grab 4th for even small hills or headwinds). Also treat redline as sqrt(0.6) * 6500 = about 5000 RPM. Don't use A/C at idle... that's 50 kPa MAP @ 1050 RPM, which is borderline even on the proper oil.

 

[deepblueQ]

And what do you tell owners that have ignored all this, and have over 100,000 miles on their engines, and still going strong...? The tranny shifts down when it determines the drive-train & engine are under excessive load (I find my cars computer has this load situation figured out vary well)...

The TCU calculates what load is acceptable according to one particular oil viscosity. It doesn't have oil temperature or pressure sensors (besides the pressure switch), so it doesn't have any clue what the actual viscosity is at the moment. If you put in 0W20 and go for a 4th gear highway run in Florida, then the viscosity is going to end up far lower than the TCU is expecting, no question about it.

I realize there are plenty of people who have ignored this and haven't blown their engines. I expect that they are not in exceptionally hot climates, don't drive it very hard, keep the revs a bit higher, or some combination of the above. Even so, I would expect that their bearings are worn a fair bit more than engines run within spec. MB has to design for extreme situations... when you put in thinner oil, you're just taking away much of that room for error that they built in.