It took me a LONG time to switch to synthetic.. but what sold me was when I bought a used Town car with 165K on the clock.. it had synthetic from new, and I had proof with all oil change receipts........ so I just kept up the same brand and type, Mobil 1..
I took the heads off at 190K and the damned thing was brand new inside... it was amazing..
I had another Town car, with the same engine (4.6) with 125K, on normal oil, but still regular changes... it was OK when I pulled it apart for a head gasket leak...... but not nearly as nice as the one with 190K...
since that day.. I use synthetic in EVERYTHING.. I mean, it really isn't very much more per year. I mean, when a tank of gas cost over $60.00... a $20 dollar bill more for synthetic oil isn't an issue...