Is there any reason I can't put a fully synthetic oil in my 05 WRX 'ppp'?

I was planing on putting 5-30 fully synthetic in my car. As my very friendly garage has this in stock and I'll probably get it free .
But after reading the manual it recommends semi synthetic. Is there much difference? Will I do any harm?

 

My car has been on f/s from day one with no probs now at 63k.

 

That's good then

3000 mile oil change for me then

 

fully synthetic oil is good for the engine. but i think 5w30 is a bit too thin, i would go for a 10w40 aleast, best is 10w50 this will protect your engine more at higher engine oil temps