+1 - Ars said exactly what I would have.
I run a 5W30 (or sometimes a 5W40 in the summer) synthetic and stick to the factory interval. The increased cost is offset by fewer oil changes...but the cost of oil is cheap compared to the cost of repairs...
2006 Corolla (kid's car - 165K, Eco Contacts, Castrol Edge 5W30)
2005 MB S600 (84K, Michelin AS3, M1 0W40)
2002 V70-XC (200K, AltiMax RT43, Pennzoil Ultra 5W30)
2002 V70-T5 (179K, IPD bars, Bilsteins, Conti PureContact, Mobil 1 0W40)
1992 MB 300E (Sportline - 160K, Yokohama Avid, Liqui Moly 5W40)
1990 4-Runner (V6, 4WD, mild lift - 257K, 31" BFG AT KO, Castrol 10W40)
1932 Packard Sedan (straight 8, dual sidemount spares, original paint and interior, Shell Rotella 15W40)