I don't think any manufacturer can 'require' the purchaser to use one type /brand of oil over another due to some lawsuits filed several decades ago (remember when Ford only allowed Formoco oil or warranty would be voided?) Several class action lawsuits later and there is no more 'required' uses of brand name oil, only API service grades and weights are listed in manuals now.
It is largely up to the owner of equipment to select the type of oil that they use whether it be straight mineral oil, regular motor oil or synthetic as long as the type is what the manufacturer lists as required.
My interpretation of 'Strongly recommended' would be "required" by manufacturer and still not be breaking the law. They could not void warranty if you used any other oil that is listed in their manual but by using the recommended oil, you WILL have better operation and possibly longer service life of the engine.
Bottom line, use whatever YOU want LittleBill21 from PA., but don't try to argue the semantics point that no where is it "required" when it certainly is recommended in many manuals.
A good example of recommended vs required would be if I recommended that you don't jump out of an airplane without a parachute, would that be a requirement? NO, you could still do it, but might not be the best thing in your interest.