As far as going to synthetic, it should be OK to change over at 20,000. I bought a 2004 last year with 41,000 miles. I changed over and I used a Purolator Pure 1 oil filter for the first two oil changes. I felt the oil was getting dirty fast. That filter is supposed to clean better than most filters. I now use the Toyota oil filters.
I first used synthetic on my 2001 4Runner and I have had good luck with that car and my 2004 4Runner. I use Mobil 1. I used to use Castrol GTX and all of my older cars have lasted a long time. I tend to go a little further between oil changes with sythetic. I usually find it best to stick with one type of oil, I even try to use the same brand and try and stick with a schedule of when you change your oil. To me, changing oil often and on a schedule is the most important thing. The main reason I use Mobil 1, instead of other synthetic oils, is that it is competitively priced, it is available at most stores and it is a class IV sythetic.