There is a shelf life for open containers of automotive fluids. Over time, engine coolant can form something that looks like crystals. Brake fluid and, to some extent, oil/transmission fluid, attract moisture so keeping them out of the sun and tightly sealed is important. According to the folks at Sta-Bil, fuel stabilizer will last a couple of years. Another issue is that the products you have may not be compatible with the car you’re driving – depending on how long you’ve had them. For example, there are at least eight different types of coolant available today; when I first started in the business, there was one. If you have any doubt about the integrity of your supply, dispose of the old fluids properly and buy new.