Is straight 30 weight the rec. oil for a Lehman. I use 30w in my DDs, not because I like it but because they will burn a lot of multivis. If I could use 15w40 I certainly would. I cant imagine why anyone would voluntarily use 30w if its not required.
Most older engines were BUILT for single weight oil , the multi weight is to give a tiny better fuel burn , as the rings have to work less in scraping it down.
DIESEL OIL is defined by its additive package , not by its weight.
In some, 40% of the oil will be chemicals to improve dirt carrying , avoid rust or to neutralize acid in the blow by.
Every can of oil will have a code , look for it , learn what it means , and use what the engine builder thought was proper when the engine was built..
On older DD 2 strokes they started with 30 wt and soon learned the engines lived far longer with 40 wt.
DD requires CFII oil only,, most 4 strokes can use any DIESEL oil that will be on the shelves today.
Most if the oil improvements were forced by the EGR choking the engine with dirty exhaust .
Thanks EPA,