I'll pass your advice onto Dad. The Merc had a new muffler approx. 8 years ago, and in that time probably travelled 20,000 miles - but we'll check for leaks.
The removal of leaded petrol from the marketplace has been a challenge to owners of older cars particularly where the cars have engines which don't have harded valve seats; in these instances you must buy an additive to add to each tank of fuel to protect the valve seats. Lead replacement petrol (LRP) came into the marketplace (as an alternative for cars which run on leaded petrol and can't/don't want to run on unleaded) but it will be removed from sale in a few years. People who use LRP pay extra for their gas - equivalent to about 20 US cents per gallon.
When my Merc was on the road, it ran on leaded petrol and it hasn't been on the road for a couple of years so I've no experience since then with the new fuel.
Thanks for your assistance, Jennifer