Oil viscosity

What viscosity is suggested for older engines, like my 73 450sl with 112,000 miles. I was using 20-50 dino, but a few days ago switched to 5-30 Mobil 1; this is what the mechanic suggested. I've noticed Valvoline markets an oil, non-synthetic, specifically for higher mileage engines (also, they market a similar ATF); does anyone have any experience with these products?

Also, the engine was slightly leaking before the change, and I think its leaking a very little bit more now. I had my heads retourqued today and I'll see how it goes.