Yes seafoam is safe to add to your engine oil and can help get rid of and prevent engine sludge and help protect your engine bearings and extend engine life.
Seafoam is safe to use in all engines oil including regular oil and synthetic engine oil.
You still need to make sure you change your engine oil within 3,000 to `10,000 miles depending on the car manufactures recommendations.
I personally never go any further than 5,000 miles on any engine oil because I want to protect my engine and the transmission fluid in my vehicles gets changed every 20,000 to 30,000 miles which also extends the life of the transmission.