I have had this car for quite sometime now, and have always put Rotella 15w-40 oil in it. Is this safe? Come to think of it, I have put the same oil in my other gasoline cars, i.e. two 2006 Ford F-150s one with a 5.4 v8 and the other with a 4.6 v8. Is there any harm being done, or any benefit.
I don’t see any harm in it but that oil is meant for diesel engines. The thing I like about cars is that you can read the instructions and do something else and nothing bad will happen, usually.
If you read the owner’s manual, you will likely find that this oil is not recommended for American use. I think you will find that 5W30 for winter and 10W30 for summer might be more suitable. In hot countries like the Middle East they may use this grade.
In the past, engines had wide tolerances, and a 15W40 oil would not cause too mcuh damage on startup, when the oil is the thickest. If you live in the Northern states, and park outside you are GREATLY REDUCING your engine life by using this oil; it’s too thick for rapid lubrication with cold starts. It also reduces gas mileage.
As others point out, that grade is normally used for heavy duty engines that start in the morning and go all day.
The only place where I would use a 15W40 in a Hyundai would be in the tropics, Hawaii or Puerto Rico, if you happen to be writing from there.
As far as you trucks are concerned I would only use 15W40 if you are towing heavy trailers. It does nothing for engine life, and may shorten it in cold weather.
Please read the owner’s manual of all your vehicles.