I 'member a fella I used to know - had a pair of 12 V 92's in a boat. He'd light 'em off an' idle 'em for maybe 15-30 minutes before we cast off, but they never got past slightly warm. We'd complain "they're not warmed up yet". Cap'n would always say "we'll warm 'em up underway". Never got very cold there anyway, with the 2-71 genny screaming away. We were kids. Under 30.
Cap was right. Absent load or pre-heaters they don't really warm up, not like a otto cycle engine will. Warming a diesel without load means getting "lukewarm" due to the internal and external friction, air drag, oil pump - that sort of load - pretty small, might be a kw in a 6-1 l'iod but I bet it's more like 300 watts. Meantime the machine tends to wetstack and carbon up. On the boat we almost never ran past 12 kts, though she'd do something like 30 or a bit more at 100%. Cap had a stack fire eventually, we were trying to outrun a storm 'bout 120 miles off Longbeach CA and Cap opened 'em up for the first time since trials..purty flames, no damage. (That boat was built for $15 per bl oil, long gone now.)
I suppose what we're really looking at here in starting is letting the engine get oil circulation going and oil into the ring lands. Past that, which I believe takes only seconds, I honestly doubt that any gain accrues. Load obviously brings increased temperatures in an Nth engine, especially so in diesels. Avoiding extremes of thermal-stress, temperature change over time, means adding load gradually. Avoiding wetstacking and carbon problems means adding load quickly. These two goals are somewhat juxtaposed. Obviously there's a compromise rate. My guess, (which seems pretty close to Skeeter's way of doing this), for my very mild ambient temperatures is start plus 30 seconds to 30% - just a guess, but so far as I can tell it has not created any problems yet. If it were freezing I'd make it 60 seconds to 15%,90 seconds to 30% . (If it were freezing on a regular basis I'd think real hard about heaters - say the hot water heater from an old trailer thermosyphoning through the l'iod. Or maybe save some heat from the previous run and valve it in a few minutes prior to start)
The Enterprise engines at Rancho Seco back in the 1980's were kept warm with an auxiliary jacketwater circ pump and auxiliary L.O. pump pushing warm fluids through 'em at all times except while they ran, then the engine-driven pumps took over. Each of those engines went from stopped to 100% (6000 hp) in 10 seconds. I thought that was a bit fast and that the engines ought to have been set up with turning gear as is found in steam turbines if one wanted to go to 100% that fast. I suppose they ought to have been dual fuel designs and then used for peaks as well as standby emergency, but engineering and regulation are very conservative arenas - that was too experimental.