Are modern engine oils suitable for older vehicles? No. Modern engines are designed to run on very low viscosity (thin) oils, such as 0W and 5w which are now common. These are much too thin for older engines, which were designed to run on 20w in the winter and 30 or 40 grades in summer. Multigrades, such as Millers Classic 20W50 mineral engine oil, Millers Classic Sport 20w50 semi synthetic or Millers Classic Sport 20W60 semi synthetic would be suitable for older vehicles.