I have looked at the manual... but can not tell if our pistons have the holes behind the rings which are common.... ?
This might provide a way for something to get back to the crankcase oil without having to get by two compression rings whose top is made so that compression forces the rings out against the wall of the bore ..

I just happened to have a 240d piston lying next to the computer for reference (true but not there especially for reference!). There are holes visible which come from the ring grooves...about 2 to 3 mm I estimate.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Since there is going to be a distinct lack of unburnt diesel compared to a gasser the additional longivity should be attributed not so much from the lubricity of the diesel (which would be a tiny part to be sure) but to the lack of washing off the lubricating oil that is deposited there as a part of the normal engine operation.

tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I agree... particularly in those gas engines which used a carb... the gas and air was totally in contact with the working surface of the rings ...
I don't know how this would apply to gas engines with fuel injection ... direct injection was mentioned... is that type made to wait until the piston is at the top of the compression stroke before gas is injected ? Or is it injected in a manner so that it will be mixed with the air before the air is compressed ?