Exhaust back pressure won't cause an oil leak. The two are unrelated. You really only need to rap the converters with a mallet, if a CAT is deteriorating, it will rattle. Although I guess out of whack temp rises & drops could point to a failing CAT, listening for a rattle is quicker.

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1999 E320 (gave to daughter) 250K
1994 F150 (may be the end of its road) 240K
1989 560 SL (Reassembly after paint job) 160K
1986 560 SL (deceased., gave it's life protecting my wife)
1988 300 TE (departed)
1994 945T (still running strong with an upgraded cam) 349K
1986 242Ti (deceased)
1968 GT500KR (under restoration)