One little thing that everyone seems to overlook about using low restriction air filters on old diesels , is that when you lower the restriction to the outside air, you reduce what little vacuum there is in the intake system. This has one nasty little side effect on old diesels in that it reduces the scavanging effect on the crancase breather. When this happens, blowby gasses arent removed from the crancase as well, causing crancase pressure to rise, which in turm can cause oil to start belching through the breather and get sucked into the intake increasing oil consumption. it can also lead to oil leaks in other places.

And they do let more dirt through which has to go somewhere. much of it winds up in your oil and not all of it will be caught by the oil filter so oil quality will suffer.

That said, I've been thinking of trying a K& N on my 300sd...Hey , at least I know what I'm in for