Two senior Yanks just finished 4 nights in Sintra with a rental car picked up in downtown Lisbon. We had a very small 2 door Fiat 500 - thank goodness - but still scary driving on the narrow lanes in Old Town Sintra where our Airbnb apartment was located. Very narrow steep winding cobblestone roads barely wide enough for our narrow car. Did I mention steep - like 30 deg incline with blind hairpin turns - thank goodness it was one way? But the lower portion of the road was a bit wider but then supposedly 2 ways.
After finding a place to park on the side of the very narrow steep 2 way portion, we did not move the car again for 4 days until we left. Which was another adventure on equally narrow steep windy roads. We were so glad to get out of Sintra without incident!
Even some of the 2 way major roads to the palaces were equally scary. Honestly, unless you are a local or from an area with similar narrow steep winding roads with blind hairpin turns - take the bus to the Pena and Moors Castles.
Just an opinion - maybe I would have felt differently in my younger days!