SAE 50 would be a bit too thick for an older non pressurized engine, SAE 30 is the best bet. I will not use a multi viscosity oil in older engines. And for that matter there are some engine manufactures that recommended against a multi viscosity oil.
Multi vis is good for brand new cars.
Multi vis oil the base oil is the low number like 10W-30 is a 10 weight oil, with viscosity index improvers to make it similar to a 30 weight at temperature. Remember lots of newer engines have such things as antifriction lifters etc and can handle the newer low vis oils with no EP additives.
