On thick oils, engine noise usually gets depressed,
And on a general note (I am not directly / indirectly pointing out towards your cousin from whom you bought this car), some people use it when they are selling their cars and engine is weak. Its known as the "Potato trick" (although experts find out this trick).
The trick involves pouring in extremely thick engine oil (may be Diesel engine grade oil) at the time of selling the car, especially when the engine is weak too. Any / all sorts of engine noises will mostly get depressed. There will be little-to-no smoke from the exhaust, and engine will seem super quiet.
Issues will only pop up, when you'll have bought the ride, and use it for some time.
