15w-40. Most brands are labelled for diesel use...

[caleboar]

But I assume they are suitable for my 76 450SE as well? Is there a harm in putting diesel engine oil into a gas engine? I can't find any "non-diesel" 15w-40.

[Ilike300sd]

Check manufacturers website or oil bottle.  Many oils will say "mixed fleet" or similar which means they are recommended for both gas and diesel.  All the diesel 15w40s I've been using are ok for gas also.  Delvac, Redline, are a couple examples.  I know lots of people who run Rotella synthetic in gas engines also with great results.  Many diesel oils have higher TBN retention, higher detergents, dispersants, and anti-wear additives than gasoline engine oils so they can be a good choice.  That's a total generalzation though and it depends on what specific formulation we're talking about.

[Adrianl]

There are plenty of non diesel 15w 40 oils out there. Don't put diesel oil in a gas engine, detergents etc are different. I know that some say they can be used for both but in those situations it's still an oil engineered firstly for diesel combustion. Though it wont blow your car up, it's just better to use a gas oil. I get 15-50 mineral oil for my 850 from Shell and 0-40 for my Porsche from mobil 1. I really like the shell oils though it's marketed as use for an aircraft.

[Ilike300sd]

...Don't put diesel oil in a gas engine, detergents etc are different.

In many cases using a diesel oil has shown in UOAs to be much better than gas oils in a gasoline engine.  It depends on the specific engine and specific oil formulation and one can't generalize.  Some engines can really benefit by the higher additives of HDEOs.  For example Delvac 1/TDT has a higher viscosity, higher HTHS viscosity, and higher ZDDP than Mobil1 0w40.  Doug Hillary, who works in deveolpment with both Porsche and Mobil, runs Delvac 1 in his Boxster instead of Mobil 1 0w40 due to the higher HTHS. 

[Gerard]

have a look for any API ratings on the labels.

API petrol grades begin with an S, like SF, SG, SH etc, and diesels go from CC, CD, CE etc.  I cant remember offhand what the 450 engine specifies but probably anything better than an SF/SG would be fine. 

Of course..things move on a lot with oils and standards.
Diesel oils have more detergents and dispersants for all the particles.