So last night I spoke with 2 engine modders - one in CO and one in CA
They both say, it is recommended NOT to use WD40 in an engine in replace of After Run Oil. The reason behind this is: 1) it is highly flammable and using to much in an engine blown out glow plugs and it can screw up the button head because it will not allown the glow plug thread to hold, becuase its used to ease the tention between screws nuts and bolts.
2)it is a degreaser/cleaner first and lube later. The oil they use in WD40 is a thin low content oil, for smooth surfaces the oil do not stick, it just slide off and settle at the bottom.