Recently acquired 500 sel had 10-30W synthetic engine oil according to p.o. It also came with massive oil leak from lower oil pan gasket. Upon replacement of oil pan gasket should I stay with synthetic oil but heavier weight 20-50W? or go back to conventional Rotella Diesel 10-40W that I used successfully in a 450sel? or perhaps try new Rotella Diesel Synthetic? Am I throwing money away staying with synthetic on a 140,000 mile engine? Recent article on this board about engine oil for older engines didn't resolve issue for me.