switched from conventional oil to synth 0w20 - questions

[josh044]

I made the switch this weekend. It's the first oil change I've done since I bought the car a couple thousand miles ago.

The previous owner used conventional oil, and the used oil coming out didn't look too nice, pretty dark.

So, a few questions...

With synthetic, how often do I need to change it to keep up the warranty (I'm at 43,000 - the previous owner had an extended warranty that transferred over to me). I've heard it's 5,000 - but is it the same if I use synthetic?

Also, with as dirty as the old oil was, do I need to do anything special to clean the old gunk out? Should I change out the oil earlier after the first switch? Should put some additive before the next change?

Thanks!