I own a '97 Max SE, 2001 SE Miata, 2002 Protege5 and a '96 Astro. I run Mobil 1 in the first three and Castrol GTX in the Astro. I change oil/filter myself at 3k (excessive, I know). I want the first three to run forever but will replace the Astro at some point when reliability gets too bad. My question, rather than put new Castrol GTX in the Astro, should I save my 3k oil change Mobil 1 (10W-30 in summer and 5W-30 in winter) from the first three and use in the Astro? Also, I'm thinking of switching to Schaeffers and a $250 order but sticking with OEM filters and maybe go to 5k oil changes. Which type Schaeffers? Thanks for any advice.