Hi Guys,
I have just had the exact same problem develop on my 300tdi (158K) - Just got back from a couple of days away at the coast. It started coming on when I dropped the revs to get down a rather steep bank. I had checked the oil and it was showing full (when cold). Driving back home it would come on when pulling up at r'abouts etc, generally when the revs dropped below 1500, but went out when accelerating away. Then,about half hour into the 2hour journey home it came on and stayed on at all revs and speeds. I pulled over, checked the oil - no probs - gave it 5 mins and started it again - went out.
I am going to try the oil light switch first which I believe is located by the oil filter?
Other than that the engine was faultless, no smell of burning oil etc.
Just as an after thought, On it's last service , 2000 miles ago, the oil etc was changed and AMTEC engine restorer was added. It has helped restore some umph to what was becoming a tired 300 before that.