I just bought a 1999 Mazda 626 with 31,000 miles on it,it's 4 cylinder automatic transmission,and the service tech at the dealer told me that synthetic oils like Amsoil are so slippery that it can get by seals causing leak and I was better off with non-synthetic and change oil every 3000 miles. Any truth to that? I did have a 1998 Honda Passport before with 133,000 miles on it when I traded,I used Amsoil for 90,000 miles,and the rear main seal did leak about a 1qt. of oil a month. I thought I would change over to Amsoil in the Mazda but now I'm not sure. I'm afriad of seal leaks. Thanks for all and any input. Robert L.