some will leak past the rings but that amount is generally insignificant except on smaller diesels. The major issue lies in the closed crankcase ventilation system. The crankcase is vented. It has to be per federal emissions regulations. Gone are the draft tubes, so initially they vented the crankcase via the valve cover and oil separator directly to the intake manifold on non-turbo diesels. So generally the tractor (or engine or whatever) gets turned over, then oil is dumped into the valve cover. With the vent tube going directly to the intake manifold, the oil that gets into the valve cover also gets into the intake manifold; and in a lot of cases, a LOT of oil (the entire crankcase's lube oil sometimes). Then as the engine is turned over via starter, oil gets pulled in through the intake valves, the cylinder can't deal with it, the flywheel weighs 50 lbs, starter has a ton of torque and boom, bent rods and broken pistons. It don't always happen but I'd say 75% of the time, maybe more often. I've even seen head lifted from the block in an extreme case. Just makes a big mess.
The tier 4 engines (and now tier 5) have a different style vent system; lot less likely but it still happens. On a DPF tractor if you get oil into the DPF, you will be removing the DPF for cleaning, or in some cases, replacing it. They do not deal well with engine oil. Plugs them quickly. On these, if it's been turned over, engine damage, it can cost a LOT of money. This is also why proper maintenance is crucial on a DPF equipped tractor.
If the engine's turbocharged, the crankcase vents pre-turbo. So if that tube from the valve cover gets full of oil, it goes directly to the inlet side of the turbo, sometimes it destroys the turbocharger, sends pieces of compressor wheel into the engine, bends valves, etc. If lucky, it might have an intercooler, so the oil and parts get stuck in the cooler-replacement only (along with the tubes and hoses).
If you do turn it over, do not try to restart without removing either the injectors or the glow plugs (preferably injectors). Then once they're out, turn it over by hand a bunch of times using the fan to spin it. You have to get ALL of the oil out. Some of them are next to impossible to get to the injectors or glow plugs, some are easy. I did a 3901 last week takes about 3 hours give or take just to get TO the injectors (and glow plugs), then just a little longer to get them out. They're all different in that sense.
