I used to go with the 3,000 mile interval, but not the “or three months” part Unless you leave your car parked at the Airport for 3 months at a stretch, it doesn’t ‘go bad’ three months after you open the bottle.
Now I go synthethic at 6,000 mile intervals. The oil changes cost twice as much, but are done half as often, and the engine gets better winter cold start performance.
Win-win.
Do you get better gas mileage with the synthetic oil?
When I used synthic I noticed the engine temp. gauge would go up to the thermostat temp. of about 192 degrees. then drop back 15-20 deg. whenn the t-stat opened and remain there unless you put a heavy load on the engine. With regular oil, the temp would go up to 192 and remain there. I reasoned that if the temp. stopped falling back with synthetic oil, then it was losing it’s quality.