450 miles is too soon, especially since the engine is an all aluminum block... with engines like these, oil doesn't get contaiminated as fast.... I changed the oil in mine right after it cracked 3000 and the oil was just only brown
the oil filter on the 2.3 is a cartridge type... I hate that s*** because it is messy and has a great chance of beine installed improperly by anyone, so I converted it to the spin-on (conventional) type... the thing is when you convert over, you need to run 5W30 oil instead as mazda japan recommends due to the fact that the spin-on filter has different flow characteristics and thus different oil pressures results in the engine... using 5W30 will maintain similar pressures.... FYI, the newer j-spec 2.3 has the spin-on filter