What would cause diesel fuel in oil?
Funnily enough I've had something similar to what Solar Mike suggests... happened with a much older, petrol engined VW Polo. ECU got fritzed by a faulty earth cable, and in the midst of trying to get it going again, before the root cause was spotted, it ended up dumping a good bit of the fuel tank into the inlet manifold, from where it leaked through (petrol - and diesel too - being quite a bit thinner than lubricating oil) the valves and then the piston rings into the oil sump. So then I had to change the oil and top up the petrol tank as well as replacing the earth strap and ECU...
It may not be the exact same cause here, but the otherwise inexplicable overflow, the appearance of what looks like fuel in the oil, the smoke, the sudden revving... sure sounds like what he suggests (an injector that failed by locking open, at least for a short period... they can have quite high flow rates when not operating in their usual intermittent fashion) would be the case.
DPF has almost no way of causing what you describe. The diesel particulate filter is usually a box living somewhere between your catalyser and silencer, i.e. halfway along your exhaust pipe, underneath the passenger cabin, a long way downstream from the engine, designed to catch soot particles in the exhaust. It has absolutely nothing to do with the fuel system or the oil. Perhaps you or other people advising you have become used to calling the general Check Engine / ECU or Sensor Fault light as the "clogged DPF light" as that's the main thing that it tends to come on for, or perhaps it's also lit up because the filter has become clogged (little wonder if the engine was suddenly overfuelling and producing a shedload of rather damp soot), but correlation in this case is certainly not necessarily causation.
I'd suggest getting a better grounding in the theory of your problem before mouthing off at the professionals who didn't cause your problem, might not have been fully briefed on the facts of the matter, and can only call it as they see it, which sounds like, at least from a very basic level, they got mostly right. (I almost wrote "but without getting the correct cause", but it doesn't sound like they even said a cause... they just said the oil level is too high, by four litres, which is quite a lot considering a typical car has about four litres of oil in it under normal circumstances ... unless you drain it out, separate it and examine it, you can't tell straight away that it's oil mixed with diesel fuel, and certainly not if you're just looking at a dipstick covered in some kind of substance coloured mostly by suspended soot particles. And that over-fill alone is enough to cause a lot of trouble, before even figuring out why it happened and how to to prevent a recurrence)
Successful Farming Engine Man Ray Bohacz has engine grease and field dirt under his fingernails from a life spent repairing vehicles and running a farm. When he's not busy in the shop, he's working on maintenance articles and videos for Successful Farming magazine and answering questions from readers. The following is a letter Bohacz received from Ryan Damery:
I have a Freightliner with just 282,000 original miles. It runs with a model 3406 Caterpillar diesel that was overhauled three years ago. I just bought the truck from my father, but I suspect that the overhaul was required at such low miles because when he bought it, the dealer sold the truck with an air cleaner that was not fitting properly. A year later he checked the air cleaner to find that it was not fitting tight and had been sucking dust for a year. A few years ago we started noticing a lack of power about halfway through the year and only had put about 1,000 miles on the truck per year. The oil level was high at that point and it smelled of diesel. I have searched the Internet regarding the problem and it seems that this particular Caterpillar engine has an injector seal that can leak. When that seal leaks, diesel gets into the oil. Do you know anything about this problem?
Response from the Engine Man
Based on your finding of diesel in your engine's oil, your theory of the problem being an injector/nozzle seal or cup is probably correct. But you, or the shop that will work on the engine, needs to discover "the smoking gun." Some engines may have a crack in their cylinder head which could cause this same problem. But I do not think it is the case with your Caterpillar engine.
Please keep in mind that diesel fuel in the crankcase drastically impacts the oil's lubricity and creates excessive wear in all parts and the turbo bearing. I would take an oil sample with the contaminated oil and send it to a lab. They will tell you what you already know, that there is fuel in the oil. But what you need to determine is if there are any high levels of any engine bearing material in the oil. That will allow you to judge if there is any real damage that has occurred to the engine. Based on the report, I would then go after the oil dilution problem. Even if the engine has seen some excessive wear from the diluted oil in the past, that Cat engine will probably serve you well for many years with the low use you give it . . . that is, once you stop the diesel from getting into the oil. Thus, do not get too upset if the oil analysis looks a little on the bad side. It is better to know a general idea of the engine's health before you do anything.