But she's not using synthetic oil, but a synthetic blend, which likely doesn't have that much synthetic oil in it. Even Mercedes has acknowledged synthetic oil's propensity to leak. They had to - it's more prone to leak.

"Mercedes engineers" don't recommend anything - they have a long list of approved oils which includes conventional oil for a car of this era. Mercedes North America recommends Mobil 1 but they have a joint marketing agreement with Mobil, which means they are paid to say that - and that sort of an "opinion" should be thrown out as lacking objectivity - tainted by financial interests and contractual obligations.

To the OP, your car isn't sensitive to oil type - synthetic, conventional, blend - are all fine. There are many examples of 300k to 500k mile engines with conventional oil. But you should strive to get the viscosity right for your climate, as your 1991 approved fluids booklet specified. You haven't told us where you live. 20-50 would be suitable for the desert summer but not for a New England winter.