Wouldn't the comment be a little softer if you just left off the first three words? Just sounds less condescending!
I apologize if being forthright is offensive. But the OPs response indicates it is the best approach, as in this example, it turns out that the Owners Manual specifies gear oil while the common advice may be contrary.
I once owned a 9N Ford which two “experts” at the tractor supply store tried to convince me to use “universal” tractor fluid in it’s transmission. (The 9N also uses the trans fluid for hydraulics.).
If I had used the ”universal fluid” the tractor likely would have failed (as subsequently I read several such instances where unknowlegeable owners had fallen for the improper advice. REprints of the original manuals are readily available and the manual specified 80/90 gear oil.... and that is what I used in that 70-year old machine per the manual because I READ THE MANUAL... (The tractor was subsequently sold after I used it an additional 15 years and is working perfectly for it’s “new” owner today.)
These discussion remain online for many years. I believe being adamant about the value of the mfr’s manuals might save another owner from making expensive mistakes. Perhaps I could soften the blow somewhat. YMMV