My Mazda’s engine has developed a rear engine seal leak at about 60K miles. It is not a major leak (uses 1 quart every 3000 miles). Last year, in May, I switched to Castrol Syntec Blend (5W-30), and 100 miles later I noticed that oil was leaking onto the exhaust, producing some smoke. I drained immediately the Castrol, and replaced it with Pennzoil High Mileage (5W-30). The smoke was gone, but I still noticed oil drops under the car. In my case, Pennzoil HM was a disappointment. One month ago, I decided to go with Mobil Drive Clean High Mileage synthetic blend 10W-30, and as soon as I did, the leak was gone (no oil under the car). 1. Why was Castrol Syntec Blend leaking so severely? Would any group III synthetic oil leak? 2. Why is Mobil Drive Clean Blend HM not leaking? Are its HM additives more effective than Pennzoil’s? 3. If I use Mobil 1, should I expect it to leak?