Regarding fuel brand -- These days, the lowest prices seem to be at a couple of Citgos. Don't like supporting Hugo Chavez, but the gas is fine. When I lived in Montana, Conoco was a good choice, but we don't have those here. I always run 91 or higher.
125,000 plus a few years ago, traded for new Avalon. It was my impression the head gaskets on those 3.8 usually failed at 80,000 or less.
There were a lot of low mileage failures on the 3.8 head gaskets. Cars driven more miles per year accounted for a lower percentage of the failures because they were not subjected to the same temperature stress.
To wit, a car driven 10,000 miles a year, for example, would have been involved in many more short trips, quite a few of those where the engine was not allowed achieve normal operating temperature. This repeated heating and cooling, and the stresses that go along with short trips, played a key part in the head gaskets failing when few people would have expected it. When I was working at a Ford dealer the early part of this decade, we had a '92 Taurus on the lot with 49K miles on it. It was clean as a whistle. But it had head gasket failure. Who knew? All its life, it just went to church, to the market, and home again. Bingo. There's the reason. This is not to say that all the little old ladies of the world are wreaking havoc on head gaskets everywhere, regardless of engine. Rather, the 3.8 Essex head gasket setup was just predisposed to failure.
If it had been owned by a travelling salesman who did 30K a year, chances are much better that the failure could have been averted, or at least signifcantly delayed.
By the way, every car in my sig requires premium so you know I'm loving the gas prices here in California.