I'd conduct my own experiment. First thing I'd do is change oil filter to what I had before. That would be easy enough. If that didn't fix the rattle, I'd change the oil weight back to what I had been using.
I was using a very well known 0W-30 in my LS400 and it rattled on start up. I kept trying to assure myself that the oil was still protecting my engine. I changed oil viscosity back to 5W-30 and it went away. I have been amazed how different oils, even in same viscosity, can make your engine run/idle quieter.
When engine metal is moving you shouldn't be able to hear it rubbing against itself.. The mere fact that it went away doesn't necessarily mean everything is "AOK." When my car was rattling on start up, my head told me the oil was fine. My head said, "Don't worry about it." On the other hand, my heart told me it wasn't alright. If the rattle keeps "knawing" at you, go with your heart.