I agree with this.Yes I would try the Valvoline MaxLife High Mileage 10W30 syn-blend next time. The MaxLife synblend has a good reputation for fixing leaks.
If that doesn't work, try the Pennzoil conventional 10W30 HM. A fine oil for a Buick a V6.
We had a 2002 LeSabre w/3800 in the family from new and all our cars at the time got Havoline conventional 10w30. At some point my dad thought he'd try Mobil 1, and it proceeded to leak. He went back to the conventional and the leak stopped. Car was sold with ~160k mi and no further problems with oil leaks. We also had a 92 LeSabre before that and it went ~225k mi on conventional before it was sold. No oil leaks.
Some may parrot that synthetic oil will only clean out a preexisting leak, and I don't totally disagree. However, I suspect seals in some older cars are not very compatible with synthetics. For your sake, I hope you're dealing with the latter and can reverse the leak.