Hyundai Service Centre Scam!

Guys! Another level scam is going on in the hyundai service centre! I went for my 3rd free service today. I was given a bill where they have mentioned they have put synthetic engine oil but when I asked the technician who worked on my car that which oil did he put, he said normal oil. I was not there at the SC when service was going on. Synthetic oil is definitely 2-2.5k more than normal oil. Synthetic oil costed me 3800₹. Finally they replaced the oil again infront of me. So this is to everyone, if possible, be there at the SC during the service. You cant trust them. God save people who opt for FREE pickup and drop!. It was an authorised SC of very renowned dealer in Bannerghatta road, Bengaluru. DM for more details. Also, Suggest how this scam can be stopped!