when the 300 was built it likely used a non detergent oil of the sb classification. That is a mineral oil with few additives. The zinc and phosphorus was added in the 80's when engines went smaller and faster. Started to come out in the early 90s to increase life of catalyst converters.
I use a good 10/30 gas engine oil and when the engine is clean go to a diesel equivalent. You are so much more likely to screw something up with a high detergent oil in a dirty engine.
the following categories have been declared obsolete:
SA: Adopted in 1971, but known as API Regular prior to then. This performance category identified mineral oils which contained no performance additives and were intended for the service lubrication of certain low-performance gasoline powered automotive engines typical of the period 1900 to 1930 in North America. Because these oils did not contain any detergent additives, they were also commonly called "non-detergent".
SB: Adopted in 1971, but known as API Premium prior to then. This performance category identified engine oils typical of the period 1931 to 1963 in North America. These oils contained some minimum level of performance additives and offered mild anti-scuff capability, some limited resistance to oil oxidation and some copper/lead bearing corrosion protection. These oils were also referred to as "non-detergent".
SC: Adopted in 1971, but known as API MS prior to then. This performance category identified engine oils typical of the period 1964 to 1967 North America. These oils had to meet the performance requirements of a new "Multicylinder Sequence (MS) engine test. The MS tests were selected to evaluate protection against low temperature sludge, deposits, rust, corrosion and wear.
SD: Adopted in 1971, and also known as API MS prior to then. This performance category identified engine oils typical of the period 1968 to 1971 in North America. The engine tests for this classification included testing the cleanliness of the positive crankcase ventilation valves during short trips and stop-and-go driving.
SE: This performance category identified oils suitable for vehicles manufactured in the 1972-1979 period in North America. The MS tests were again upgraded to evaluate high temperature oil thickening.
SF: This performance category identified oils suitable for vehicles manufactured in the 1980-1988 period in North America. Once again the multicylinder tests were upgraded, adding evaluations particularly appropriate for smaller, higher revving, higher operating temperature engines.
SG: This performance category identified oils suitable for vehicles manufactured in the 1989-1992 period in North America. Oils meeting this service category provided improved engine cleanliness and wear protection for both stop-and-go driving and high speed highway service.