I recently changed my oil to Amsoil 10W-30 from the S2K 0W-30 on my '01 Honda Prelude. I use the Amsoil SDF-20 oil filter. The car runs well, but the oil pressure keeps dropping from 71 psi to 61 psi at 77mph (125Kph), and then stays constant at 61 psi. Any ideas of why this would happen? It happens when the oil is fully warmed up, not as it's warming up (that would make sense). I'm not losing any oil, and the ONLY thing I did differently was overtighten the oil filter by quite a bit, because the Amsoil filter gasket is very tough and my oil leaks if I don't tighten it a lot.