These are indirect injected engines. When you pour oil into it, most of it sits in the prechamber for a bit. By time you get the glow plug back in, some of it has dripped down into the cylinder. If you're lucky a valve will be open and it'll blow it out. If not, it'll make a half stroke and, well, it won't compress oil....something's gotta give (usually the rod, though I've seen broken crankshafts too). You put, 7 or 8 cc of oil into the engine, piston is 3/4 the way down the cylinder on the intake stroke. Put the glow plug back in, hit the key, there is enough inertia from the weight of the crank flywheel and accessory drive such that you may not even know it happened. That is exactly what happened in my case. Never even knew it but sure enough got a weak/smoking cylinder afterwards.
also putting oil into a diesel cylinder is not a good idea on account of it being a compression-ignition engine. It can try to start, it will run on oil. So if you are doing a compression test, because the engine can ignite the oil, it throws your readings way off. I actually destroyed a gauge doing this...
Did some Daihatsu stuff-, the morons who built/designed the motor actually put a vent from the valve cover directly to the intake manifold, that ran kinda downhill. If you filled the engine too quickly while doing an oil change, the valve cover area would fill up, some of it would overflow through the vent and into the intake. I had to replace several crankshafts, and lots of rods due to this issue. I caught a couple of them before oil got into the intake, and learned what was causing it.....worked with the manufacturer and they changed the vent tube shape such that it has a bend in it which must face upward (toward the top of the motor) so that when an oil changer pours it in too fast, it will run out of the fill hole long before it ever overflows through the vent.
I have been through this. It doesn't take much to cause bent rod. Remember, I learned the hard way on almost everything that I know, which may not seem like much. Maybe it isn't but hopefully it's enough to help y'all. Or maybe the rod was already bent. Regardless, it's gotta be fixed. When I see a bent rod I assume that the piston crown is compressed, the pin can be compromised as well so I always recommend checking the integrity of the cylinders (round, no taper, within spec), if ok then replace the rod, piston, pin, and rings and hope the crank ain't twisted. It's not common with kubota as it is with Diahatsu.