I agree with you 100%. The recommendation for brand and type (not weight) are typically based on relationships of vendors and money. So going to full synth or one with additives, etc can have benefits.
But when it comes to the actual weight, since all brands have weights of every spec, the recommended weight from the manufacturer is typically the recommended weight from the engineers. Most people assume a higher weight is better which is not the case. Engines like the Triton and BMWs typically run lighter weights by design.
IMO the rule of thumb is
- Run the weight recommended by the Manufacturer
- Run the brand & type based on what you prefer and what others recommended especially if it has a history of performance for a particular engine.
The big thing people overlook is warrantee work. Your engine runs into an issue and they run a sample of your oil and figure out you’re running the wrong weights. They can easily deny a claim.
5w-30 vs 5w-40 isn’t a big deal but why change the weight? Thicker at operating temp isn’t helping flow or lubrication if the engine isn’t designed for it. I agree with sticking with the manual.
this is the same argument people who think 93 is better when the engine recommended 87 octane. Might not hurt but probably doesn’t help.
(Side note is the 3.5HO does recommend 91+ for optimal performance)