I think you need to do a bit of research into this subject yourselves and not just believe what the dealers say.
5w30 is thinner when cold (first number) and thinner when hot (second number) than 10w40.
That said I think both oils suit the 1.8T, I have used 10w40 since I bought the Golf and had no problems and others have always used the 5w30 and also had no problems.
I used the 10w40 as I was recommended to by a friend who works for VW/Audi so I knew I wasn't being fed lies lol