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

It is fine as long as the oil that is in it is synthetic, but not the entire capacity of 5 qt. if you need multiple quarts put one 5w-30 and one 10w-40 you need the full range of 5 to 40.