Is it ok to put Mobil 1 5w30 synthetic oil on my 03 jetta 1.8 due to I'm unable to find mobil1 5w40 synthetic?

I'm surprised your owner's manual calls for 5W40. I would recheck your owner's manual and your VW dealer's advise. Synthetic oil meets all your VW specifications, but it's a mystery why a href="http://www.mobiloil.com" rel="nofollow"http://www.mobiloil.com/a doesn't have the exact weight oil you need.