I been using Mobil 1 5w40 for a long time, but I'm spending $ 81.00 on oil (Mobil1 0W40 this is all they had at the store) plus $12 on a oil filter. thats $ 93 bucks, is it recommended to use mineral oil or does it have to be synthetic?

Also right now I'm changing my oil at 4,500 miles intervals. Is that right? and If I was to change to mineral oil how many miles should I make the oil changes? Thanks.

__________________
1999 E300 TD 171K Miles
Winter Garden FL