My reference to using a conventional oil like GTX 20W-50 was with respect to your post that the car "has been using oil for years"
with "no signs of leaks under the car or on the ground."
My rationale was that it's a waste of money on an older vehicle (100K+) to now start using a more expensive synthetic for the first time such as 10W-40 or a 20W-50 synthetic.
After talking with her about it she said the car has been using oil for years but not that much. I checked for leaks and found no signs under the car or on the ground, also checked the plugs (prob 50k old) no sign of any burning. I changed to regular 10-40 it better but still using oil.
The reply by the Dealership Tech when you mentioned changing to 20W-50 with him saying: ďABSOLUTELY WAY TO HEAVY ... the top end would suffer"
is typically right on. According to most any Dealership a 20W-50 serves no useful purpose as far as their concerned.
So, this is a question for those more knowledgeable garage logic mechanics: Under what conditions would a synthetic 15W-40 or 20W-50 synthetic blend be a more practical advantage in a high mileage (100k+) gas V6 or V8 over that of a less expensive conventional GTX 20W-50 ?