If you have visible sludge build up I'd manually clean the internals and start with the proper weight detergent oil. If you get a clump of sludge stuck in a gallery or passage or worse, smeared on a bearing surface bad things will happen. The GOOD part is that the rockers are the last parts to be lubricated, so the sludge will most likely fall into the crankcase. The BAD part is that the oil pump won't care that it's sludge and pump it back through the system.
Run 20 or 30 weight detergent oil. Multi weight is fine... 15W40 is much to thick IMHO. 5W20 or 5W30 unless you are in the desert/temps over 100F. Its all about start up wear and getting that initial boundary layer of oil on the bearing surfaces. This will explain it all:
http://www.upmpg.com/tech_articles/motoroil_viscosity/ My experience is diesel fuel is a horrible solvent, ditto for kerosene. It's oil. The additives may do some light cleaning, but not much. The fact that when it seeps out of a motor its HOT probably does more 'cleaning' than the oil itself. And you surely don't want to add it to a crankcase. Yes, way back manuals would tell you to use kerosene (10% or 1 quart in place or regular oil) or even mineral oil.. but there are MUCH BETTER solvents that are available... SeaFoam is probably most cited.
