0W-40 vs. 15W-50

[rrodz]

I have used mobil1 15W-50 all along on my '99 ml320, now 72K miles. Now that 0W-40 is more available, is there a problem if I change now to 0W-40? Oil leaks? Any other concerns?

On my '00 C280 (49K miles), per the dealer, mobil1 0W-40 is being used all along. Now I will start doing oil changes my self using same.

Both my ML and my C do the few clappings ( tac, tac, tac for the first second or 2) on cold starts since day 1. The C does it to a lesser degree though, perhaps a smaller engine?

Please advise your opinion.

Thanks,

Rene