Wow ive opened a can of worms with this one

Simple facts guys.
Mate gave free MOT - Topped up with semi-sythetic oil onto of my existing fully sythetic 5W30 oil = Wrong and needed sorting.
Main VW dealer - Had car to recifiy above problem with oil / filter service, invoice of work showed 5W40 which is fully sythetic just not the 5W30 oil I asked for.
VW dealer IMO did a poor service in other respects, didnt get my moneys worth at all, convienced / reasured me enough the correct oil was put in, but should of had to in the first place. This amendment is also coming in the post in the form of an adjusted invoice, IMO covering me for any related problem, plus it suggested the oil test, so I am sure they put the correct oil in.
Wouldnt use Wolverhampton VW again, and feel free to copy this to the other forum.
Mike: You cannot mix fully sythetic oils? ie mixing 5w30 with 5w40? im new to all this oil lark and interested, surely they are viturally the same as the 30 / 40 numbers represent the viscosity at high running temp of 100 degrees?
Thanks to all for their comments, steve, a1mw, mike etc