I do not think that is correct Steve- the oils that contain friction modifiers are all 30 weight (or Xw- 30 multi- weight) oils as far as I know. Diesel oils are not energy saving and do not contain friction modifiers, again to the best of my knowledge. Diesel oils are all heavier than 30 weight, at least the multi- weight diesel oils are. I have seen straight 30 weight diesel rated oil but then again, no straight weight oil is energy saving either to the best of my knowledge.
Diesel oils seem to work well in motorcycles and are very popular with no nasty side effects that I am aware of, although I do not care for the way my C-14 shifts using 5w- 40 Rotella (the new stuff, the older stuff was better in my experience).
I see all these claims of clutch damaging "friction modifiers" but in all of these threads, have yet to see someone actually post an example of a clutch destroying FM in a 5-30w oil that is not in any bike specific oil. Can't say that moly is the culprit as Torco Motorcycle oil has one of the highest levels of moly of any oil and you don't hear of people complaining of clutch problems.
Not all diesel oils are 40w. As i pointed out, M1 5-30w ESP meets VW504,507- those are more demanding specs than the Rotella 5-40w can meet.
The biggest difference between diesel oils and car oils is:
1) Higher level of detergents (to deal with soot).
2) Higher level of ZDDP (this is what the bike's tranny likes).
I guess my point is that there are cheap diesel oils you can use in your bike, but these oils (I am talking about the Rotella twins) are not the best diesel oils out there,
not by a long shot. However, spending extra $$ for the better diesel oils defeats the purpose of buying them- which is to save money over buying bike specific oil.
I think in my neck of the woods, that M1 5-40 stuff is over $7 a quart. For a little less than that or the same price, I got Redline 5-40 pao/ester motorcycle oil.
As for weights, well, I have seen more than one oil analysis that showed Rotella 5-40 to sheer down to a 5-30 within 2k miles (the VI didn't hold up). Yet, the wear analysis (even on a ZX14 with 5K miles on the oil) was still pretty good.