I have a '95 Chevy Silverado pickup, C2500, with the 5.7L V-8 and auto tranny. It's ran it's entire life on dino, currently Pennzoil 10w30 (OEM-recommended weight). It gets 5K - 10K miles per year usage; it now has 80K on the odometer. Is it a good idea to change to synthetic, or should I continue with the 3K/6mo. changes with dino?