I would imagine it doesn't have nearly as much dispersant as HDEO, since LNG burns much cleaner than diesel. I don't expect it has as high a TBN since LNG doesn't contain as much sulfur as diesel. Additives to protect against fuel dilution are also probably not necessary.
Bottom line - you can probably use 15W40 HDEO in a LPG engine, but I wouldn't use LPG engine oil in a diesel.