At one time I was banished to the back on any green lane runs. The engine (2.5na) was "well worn" and when running at low power would "oil up" in the cylinders - then when I did apply power, the oil would burn off which at times could look like I was laying a smoke screen
Same thing with an old tractor (MF35) I used back when I used to do farm work. I'd been mowing thistles with a cutterbar for a couple of hours which wasn't exactly a high-power operation, and then set off down the road for lunch. I didn't think too much about it at first, then I noticed the black smoke from the exhaust (as the oil burned off) and looked behind me
Basically, when running at low power, the cylinders cool off and there isn't enough heat to burn off all the oil that makes it's way past the rings - or down the valve stems. So the lower the power, and the more worn the engine, the more oil builds up on the cylinder walls just waiting to be burned off when the power is applied and things get hot again. As mentioned, a dribbling injector will also do it.