Just want to share my experience with you, I switched to Mobil-1 5W30 when my W124 400E was 10 year old at 200k km, before that it had been using dino. There is some slight oil seeping from camshaft seals, but it has been there even with dino, I have driven 7000 km since I switched to synthetic, didn't notice any oil loss.
Not sure about your 28 year old, it may use different seals and gaskets, the result may be different.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.