The recommended frequency to top up engine oil is to check the oil level every couple of weeks (or approximately every 1000 miles) and top up as needed. It is especially important to check before long trips. The exact frequency can depend on factors such as the age of the vehicle, driving habits, and environmental conditions. Many manufacturers recommend changing the oil every 5,000 to 7,500 miles, but topping up may be necessary in between if the oil level drops. Older cars or those driven in severe conditions may require more frequent checks and top-ups. Regularly checking the oil level via the dipstick is essential to maintain optimal engine performance and prevent damage from low oil levels.