I remember seeing that name for a sound -- "butter strings" -- years ago when I was playing around with a Yamaha keyboard in a music store. Not sure which one it was -- likely either a PSR-S650 or an MM6. But I remember that it sounded like a basic string sound with a phase shifter effect added, as it had sounded very similar to a sound I had generated that way on a PSR-520 that I used to have a long time ago. So, for people who want this sound but my not have it directly on their keyboard, if you have a string sound and DSP with phase shifter, you should be able to get it -- or at lease something very close -- that way. This assumes, of course, that "butter strings" is still the same type of sound I heard back then.