How much does viscosity matter?
 

I was told to use 15W-40 oil in my SEC, but could only find 10W-40 or 20W-50. Is that close enough? What difference does that make?

I don't think it matters all that much. I run Shell Rotella 5w40 synthetic diesel oil in my 300E and its quite happy.

All 3 should be fine, but it's not that hard to find 15-40, unless you live somewhere with no WalMart. Whether you go 10-40 or 20-50 depends on your weather there in In Limbo.

Well, I tend to think that regular changes are more important that the specific weight, but I know some people here get very excited about oil . . . . I think I'll put the 10W-40 in. I also use 5W-40 Rotella on Max, my 300TD wagon.

There is a recommended oil guide in the owner's manual - pick an oil that fits your temperature range. I can say without looking and with 99% confidence that 10W-40 is OK for your engine. If you live a colder environment, the 1st number can be lower (eg 5W), to ease cold starting, and if you live in a hotter place (Limbo is probably warm, given its proximity to hell) you can go with heavier viscosities.

That's what I figured. I think the 10W40 will be fine, too. It's fairly moderate here in Limbo, 32-85 degrees for most of the year. It's about like the weather in Northern Virginia . . . .

Change often and mind temperature to appropriate viscosity. I have used mineral and synthetic oils to no ill effects but I am like to change my oil on an overkill schedule. It's also fun!

From what I've been taught in automotive maintenance classes, and from picking the head of a friendly ASE master mechanic (Dad...) the viscosity is mostly just to accommodate different temperature ranges.
Some manufacturers of engines have shifted what their engine requires for said ranges, usually down in viscosity, seeking better mileage, but it still varies by temperature.

The thing that makes a difference between oil for diesel cars versus gasoline ones is the additive package in the oil.
If you look at the back of the bottles, there should be a circular logo with a two letter rating in the ring around it. A letter code such as CF or CH is for diesel engines. (as far as I've ever been able to tell, the C first letter stands for compression ignition)
On the other hand, gas engines are covered by S* oils, which were down to SM or so, the last time I looked at one.

I don't know why, but I have only typically found both ratings on 15-40w oil, but you can occasionally find C rated oils in other weights.


That all being said, who knows just how well some of the new synthetic oils hold up?
M1 doesn't seem to rate their oil with the same rating scheme, and I've never actually seen a bottle of Amsoil, so I can't say there.

More viscosity isn't always better. In general the higher the viscosity, the larger the load the oil layer can handle. But, at the same time, the higher the viscosity the slower the flow of oil, meaning that it will have a tougher time getting into faster moving parts and with carrying heat away. Like everything in life it's a trade-off.

15W-40
 

Mind the MB spec requirement rather than the specific viscosity. If you want to use the 15W-40 oil (I do in my 560SL - probably same basic engine) you may be looking in the wrong place. 15W-40 is generally a diesel oil. There should be plenty of it around almost anywhere. My choice is a synthetic (AMSOIL). Your choice may vary.

Yeah, that's what I nnoticed when I stopped at Auto Zone. The 15W40 was available for diesels. The gas formula oils were 10W and 20W.

15-40 "for diesels" is what you're looking for, works extremely well for gas engines too.