Bob,
I dunno. A 660 for a 60" bar? I only had a 660 briefly, but my (stock) 084 works a bit on a 60" bar, especially milling! Your experience far outweighs mine, but I'm surprised. I will grant that you said "not too often" on that 660.
To the OP, if you get into milling, and continue down that path, you may find, as I have, that the size of the trunks with which you are presented tends to exceed the size of the mill/bar you have. I have three: the small log mill, which serves me well with a variety of woods and situations, a 36", and a 72" (given that I simply convert one to the other with the width bars, etc. Bars: 36", 42", 60" (and a bunch smaller).
Regardless, read Bob's stickies and remember auxiliary oiling, raising the end of the log for a gravity boost, don't forget the little stuff like wedges, etc, and chain tension! Milling is a whole different beast relative to cross-cutting.