"partly correct" .. ?
it is correct. your rear main leaks because the esters have cleaned the dino debris from the surfaces and revealed a worn seal.
the thicker 50wt will mask the damaged seal much like the dino probably did previously with its deposits.
if it was me, i'd replace the rear main, and use the M1 10w-30.
my cars never leak. i do a initial treatment of auto-rx with dino penz 10w-30, and then go to the M1. with cars over 100k, there is slightly longer treatment plan, but again..no leaks.
for the record, you should be fine with 10w-30 M1. I prefer that over the 5w-30 as its TBN seems more stable over the duration of the extended drain interval.
1999 Buick Regal LS
ZZP 2.5" SS 304 DP, GTP Cat Back , GMPP Handling Kit, poly links and midpoints, HIR 9012/9011 with upgraded relay harness, 5000k HID fogs , 12" Impala/MC brake upgrade, Mag cat 94106, Ported and Polished LIM