merc sprinter 311 cdi.....any help!!?

i have a 2000 311 cdi sprinter with 2 faults,

1)the oil light comes on with and goes off as normal when started but then comes back on any any point upto a minute once its been running,level and pressure ok....is it the sensor in the sump as cant find any other oil switch??

2)the turbo does not seem to boost as it should,it seems to suck in too much air via the filter(really loud)and it smokes black smoke,it runs smooth,doesn't cut out,no edc lights showing but seems it wont rev beyond 3000rpm, i have had a look but not scanned as no lights showing(although i intend to)but not all the time....it will do it 9/10 times but when it dont do it it runs/pulls mint as it should do!!!!???.

I have checked it over,there was too much oil in it so may explain the black smoke as it passed thru to the turbo via intercooler(breather on r/c ok),i also changed no' 1+2 injector seals as no' 1 had started blowing but no carbon build up etc, has anyone else had these faults or could they be connected or just coincidence,dont mind fitting new turbo but dont want to waste my hard earned pennies if its something simple???.

Thought i would ask cos got an answer before with my battery problem and it was spot on...thanks.

Thanks in advance for any answers, rich.