Some vehicles can use different grades, but some can't. Mine has the grade stamped on the oil filler cap, so that is all that should be used.

If the grade isn't right you need it changing, you paid for a service in line with manufacturers specification, if it isn't 0w20 and it should be you haven't received what was paid for.

If you're not seeing any problems, I wouldn't worry too much either way

The simple difference in viscosity alone between 0/20 and 5/30 will be negligible.

Unlike the difference in price!

I can't speak for MG, but sometimes oil specs change after long term testing and feedback from owners and dealers. My car doesn't use the same oil as the now out-of-date drivers handbook recommends. It also doesn't follow the 20,000 miles service interval.

<< Modern turbo engines are fussy about oil. >>

Maybe my car isn't modern enough, but it has a turbo and the handbook advises three different viscosity grades as being acceptable - i.e. their use will not invalidate the car's warranty. A range is advised for use in different climates, so the important variable will be engine wear during start-up.