Seriousl?. Of course the car will seem to be doing well, right until the point where it doesn't. The car will run great even even if the oil isn't of the correct viscosity any longer but one day the car will suddenly no longer be able to drive because the engine internals will be fused to the block because the oil turned to crap/sludge or there wasn't enough in there. And then you are stuck trying to replace a mercedes engine on your own dime. Maybe an old civic or corolla would have been a better automotive choice? Is this your first car?