Best way to do this trip is as follows. Trust me i do it frequently .

Drive on the main road Abu Dhabi - Al Ain - Khatm Al Shikla (Shakla/Shiklah ) Border - Go around 35-40 KM, Take Muscat Salalah Exit [Muscat Express Highway] - 250 Km to Muscat. Believe me it less than 500 KM

From UAE Border Oman border customs/passport is a long way back from the actual border. Stop at window, handover passports, get Exit Stamp paying AED 35.

You will also receive a piece of paper with the number of people in your car. You hand this in at the last gatehouse which is a few hundred metres further down the road.

Border post is now straight ahead. Park your car just before the window for passport control. (Or drive to window, show your passports, and park on the right immediately after the window)

If there is no queue , simply present your passports at one of the desks and get your passport stamped paying 5 OMR

They will give you the piece of paper with number of car occupants

Walk back to your car, drive to next section which is customs check their car. After that straight to Oman 🇴🇲