You might want to try a 5w40 or 10w40 to slow that leak on one OC as a test, I'd also go with a dino oil and just change it every 5k or less.
I was told when I had my 1995 Deville that the Northstars were prone to losing more oil with a synthetic, something about the aluminum block being a bit...
Rotella T6 5w40 saved my marriage
I'm running it currently in my 2003 Toyota Highlander V6, engine runs smoother and quieter than the previous 5w30 syn. Though It's 4qts of Rotella and 1qt of Castrol 5w20 syn, what I had on stock.
Next OC I'm switching over to Delo 400 5w40 syn. It's still...
Rotella T6 5w-40 synthetic, if you shop around, you can get a gallon for around $20.00. O'Reilly Auto often has it for $19.99 on sale. Amazon is about $23.00, as is Walmart.
Diesel oils are said to have a much more aggressive 'cleaning' package, to keep the soot created by diesel fuel...