The guy I bought my 1996 1.9 TDI A4 used a semi-synthetic 5w30 oil.

The engine has 175K on it and is very smooth and strong.

However, I was wondering about this oil - is it thin enough to get to all those hard to reach places?

Would it be wise to change oils? If I decided to do this, what oil should I move to that would not be too different to the semi 5w30?

What do people think is the best oil to use in this diesel engine? Thanks.