1999 Hyundai Elantra 2.0L. Oil & filter changes every 3000kms (1875 mi). After the factory fill, it had 2 OCI's of synthetic, and since then, nothing but dino. Currently have 39,000kms on motor, and I'm thinking of switching back to full synth and going to a longer 6000km (3750 mi) OCI. Should I do an Autorx of the engine before switching to synth or do you believe my short OCI's on dino should have kept my engine relatively clean?