I recently bought my friends Lexus SC400,nice car, in excellant condition. It has 85,000 miles on it. He had used Castrol GTX, since it was new. I used Auto-RX as directed with the Castrol. I then changed to Mobil 1 S.S. 10-30. The problem is it now uses about a quart every 1000 miles were as with the Castrol it was using about 1/2 a quart between oil changes, every 3,000 miles. Any ideas? Switch back to the Castrol or use another syn?