A curious thing occured when I changed the oil in the mother-in-laws 01 Tacoma 2.7 4 cyl. I had been using 5W-30 Val Max life semi syn for the last 25,000 miles and decided to try Castrol full synthetic. SAme filter (PURE 1) as always. About 3 days after changing, she calls and says the engine is noisy. I have her bring it over and sure enough, there is a moderate "ticking" in the top end. I rechecked the level, perfect. She says it is louder when cold but never does go away. I drained the crankcase, refilled with Valv Max Life (no filter change) and PRESTO, noise gone!! IS this weird or What? Anyone else ever had this happen? Truck has 64,000 miles on it. Manual says nothing about synthetic, good or bad. Just curious.