I believe the owners manual calls for a 10W30 for 113s. But after 40 plus years, a little extra film strength is warranted.
For those that are interested, new Passenger Car Motor Oils (PCMO) have evolved away from the anti wear protection that is offered by ZDDP (Zinc dialkyl dithio phosphate) due to problem related to catalytic converters. That doesn't mean that there is no ZDDP or that alternative anti wear additives can't provide the same level of protection.
For older or high performance engines that are not worried about catalytic converters, look for a good brand of Heavy Duty engine oil (HDEO) The difference can be noted by the specifications. "S" in the API approval (current approval is SM) stands for "spark" ignition. HDEOs are identified as "C" for "compression" ignition. Many of todays HDEO also carry PCMO approvals. The key factor with HDEOs is that they are required to pass a valve train wear test to meet the specification. So HDEO have a "full treat" of ZDDP. The latest HDEO spec is CJ-4. I would recommend a 15w40 or 15w50 with a CJ-4 API approval. You could also go with a 20w50 (like Valvoline 20w50 VR1) that will say something like "meets the requirements of SM" It will not say "licensed SM"
As for oil treatments, use your best judgement but there is a lot of research that goes into developing todays motor oils. Dumping something on top of a well engineered oil formulation would not be my choice. I can assure you that none of the major oil additive manufacturers (Oronite, Infineum, Lubrizol or Afton) recommend "top treatments" for their formulas.
Oh, and, "synthetic" vs. "conventional" is a whole different topic. Mobil lost their law suit trying to protect their Mobil 1 PAO formula when the courts determined that the industry does not adequately define the term "synthetic" Today's synthetic products can be made with PAOs, Group II or Group III refined oils.
Norm