I had similar issue earlier this week, I discovered two of the three screws were rusted solid, and to top that off the fixing lug holes on the box lid had been forced and were broken which meant the lid was not actually fixed properly and must have been causing a massive leak.
Really through sheer determination and lots of grunting I managed to remove the two stubborn rusty screws using some small monkey renches and remitted new screws with some washers to hold the lid down tight.
I had also changed the oil and filter and fuel filter too along with upper turbo hose.
Improvement a cross the board was immediately noticed.
Next step is to try nd sort the rumbling alternator pulley lol