My last oil change I did myself. I used castrol part synthetic. Not realizing "PART" on oil label. Next oil change is in 7,000 miles. Keeping the castrol brand. Should I go with FULL synthetic? The next round(s). 2005 325i with 135,000 miles. Transmission and engine purrs like new. 3 more car payments left and hope to get another 100,000 miles out of her with no issues. Other than standard wear and tear.