That is a bizarre question - Put the right oil in the car - OK
1.8 turbo petrol, So you want to get 100bhp per litre and put in ordinary mineral oil in it

? This is not an exceptionally strong engine, so needs the good stuff.
Fully synthetic 5w - 40w
ditto 5w - 30w
Castrol SLX, Shell helix, Mobil 1, all good quality brands. Grade 0W - 40W not so good on high mileage older engines in my opinion.
Has to meet, or exceed VW spec. which is in the service book;- VW 502 00.
Don't mess about with cheap oil, Your oil is the life blood of your engine.
If money is a problem;- the best deal is to go to GSF and buy Quantum 5w-40w fully synthetic 5 litres, plus a genuine VAG oil filter in a different box.