There is new (chemistry) technology to clean the piston ring-lands. Carboned-up black rings is the usual cause of sudden oil consumption. The rings aren't moving properly in their grooves. The new chemistry is "Valvoline Restore and Protect" motor oil (use 5w-30 in the summer, and 0w-20 if you live in a cold climate in the winter), from Amazon or AutoZone or mail-order from Walmart or Advance Auto Parts too.
Note it is a relatively new thing to be able to clean carbon off pistons to reduce oil consumption. In the past, oils could only hope to keep the plaque accumulation down, not remove it. Note "sludge" is a different animal, not the same as the crap sticking to ring-lands on pistons, and all oils deal with 'sludge' just fine, just to avoid confusion.
Background: Valvoline successfully solved bad oil consumption on big-rigs using the Cummins ISX15 engine with their previous 'special' diesel motor oil meant for that fleet diesel crowd, and now, years later, they seem to have gotten the piston cleaning tech to passenger gasoline vehicles. It's real intellectual property.
BTW: I agree with those above that said the mechanic's advice was bogus.
If you don't want to try the "Valvoline Restore & Protect" motor oil, then another interesting and proven way to clean pistons comes from HPL (pricey, shipped-only). Links below.
Superior aniti-wear formula reduces damaging wear and increased engine life. Protection at startup and low oil temperatures. Film strength that maintains stability at high temperatures and with fuel dilution. Low friction coefficients reduce parasitic loss and increase performance. Increased...
www.hploil.com
High Performance Lubricants’ premium passenger car engine oil is a multi-synthetic high performance motor oil made with the best choices of additive chemistry. When compared with our standard PCMO Premium product line this series takes advantage of an advanced VI Improver that has a superior...
www.advlubrication.com
... or just put in their chemical cleaner: HPL EC 30
Engine Cleaner
and add that to any motor oil, like SuperTech oil from walmart is fine.
Valvoline Restore and Protect,
easier to get at Amazon or Autozone or Walmart online mail-order or Advance Auto Parts.