Hello,
Not really enough information here.
I assume it's a TDi??
The symptoms your describe sound like oil starvation,this is not a good thing.
Assuming the switch is not faulty,the oil light should never come on when the engine is running.It indicates that there is virtually no oil pressure.
A good supply of oil under pressure is required to keep the moving parts away from each other,without it, rapid wear will take place.This,in turn will further reduce the oil pressure and soon a major componant will fail.
there is a funny squealing noise from the turbo
This may be as a result of the bearing running short of oil due to the lack of oil pressure.The turbo needs more oil than any other part of the engine and is often the first major componant to fail.
A few questions.....
The oil strainer is in the sump,did you take that off to clean it??
If so,what did you find,was it blocked,was the sump full of debris or sludge?
Is the engine very high mileage??
Does it smoke from the exhaust?
What condition is the engine oil in?
Why did you have to replace the gearbox?
I would have thought that at the very least you need a new oil pump.but it may already be too late.
If the rest of the engine is badly worn,the increased oil pressure from a new pump will be lost anyway.
Regards,
Nick.