There are some things you shouldn't do, which you do anyway, knowing it's wrong, like smoking and eating unhealthy food. Sometimes you do it because you find a loophole in a rule.
For instance, it is illegal to pour used engine oil down a sink drain, even if you follow it with detergent, but if you get the oil on your hands, and wash your hands with detergent, it's okay to rinse it off down the drain. Also there are some things you shouldn't do which you do anyway because it's impossible to comply. For instance, you boaters have seen the signs demanding that you make no wake. It is impossible to move a boat without making a wake. You can anchor a boat in a current and it will make a wake. Other boat signs say things like minimum speed, or slow speed, but they don't tell you how fast is too fast. They define this as minimum controllable speed. Some boats require more speed through the water than other boats do, to be steerable.
So the boats that are hardest to steer can go the fastest? How does that make sense? They make us all into outlaws. If boating sins are gateway infractions, robbing banks may be next. They don't even have signs prohibiting bank robbery, so how bad can it be?
Back to cars. We once took in a Ford van with a severe vibration in the engine. We found that it had about three gallons of oil in the engine instead of the usual one gallon. This makes the pistons strike the liquid oil and causes the vibration, and that could cause serious damage. The man said that since he knew the engine burned a lot of oil, he decided to add one quart every day before he went to work. The moral of this story is don't add anything without checking the level first.
A car comes in with a sinking brake pedal. We find the gasket to the master cylinder puffed up and springing out of position. We know what causes this. It has happened many times. Someone has added an oil based product, either engine oil, or transmission fluid or power steering fluid to the brake system. Brake fluid is an alcohol based fluid, and the rubber seals are formulated to hold alcohol, not oil. Oil-based fluids will attack this type of rubber and destroy it. This often results in damage to every seal and hose in the brake system, which can easily cost over one thousand dollars to repair. The moral of this story is don't add anything unless you know what to add.
The lady said her car was steaming from the front when she went to work, so she drove it to us after work, and it steamed the whole way. The good news was she got to work and back without having to pay for a cab or a wrecker. The bad news was what started as a leaky radiator problem, costing around $200, was aggravated into a blown head gasket, costing over $1,000. The moral is engines don't like running without water and, if you make them do it, they will punish you like a red-headed stepchild.
An aerosol can of belt dressing, if applied to your serpentine belt, will make it sound like there are a dozen tin cans tied to the belt, being banged around against the engine as the belt turns. The stuff was designed for "v" belts, not serpentine belts. If you put it on, nothing will quiet the belt down except a new belt. The stuff should be banned. Don't use it. Good belts don't need anything added.
Maybe there should be signs posted about these things.