If the color of the motor oil has turned black or dark, does it mean that it's contaminated or already ineffective as a lubricant? Should I worry about it?
This is quite normal. You shouldn't actually be alarmed that your car's motor oil has turned darker. Contrary to your apprehensions, the darker oil is actually doing its job well. It has turned black because of the dirt, deposits, and other contaminants it picks up around the engine. This doesn't automatically mean that the oil must be changed. You must stick to the recommended oil change schedule by the manufacturer. Check the fluid level regularly. Otherwise, oil can eventually oxidize due to heat and stress. When oil has oxidized, the fluid will thicken, and this won't be good for the engine as this will restrict the flow. The moving parts in the engine won't be properly lubricated and cleaned as a result, leading to wear and extensive engine damage if left unchecked.
I found a can of motor oil in the garage. I was wondering if I can still use it for the oil change. Or is this a bad idea?
If the can or bottle is sealed and it's properly stored in a cool, dry place, then it wouldn't hurt to use this for the oil change. Just make sure that it's not contaminated with dirt or water. Also check if the motor oil meets the requirements of your vehicle in terms of weight or viscosity and other specs. Check your vehicle manual to be sure and also read the labels on the can or bottle.
Is it true that low battery voltage can lead to low fuel pressure? I had the fuel system of my vehicle checked by a friend who's a home mechanic. He said that the fuel pump runs slow because of the low battery voltage.
It's possible since the low voltage has an effect on how the fuel pump runs or performs. With low power, it may not deliver adequate amount of pressure. In some cases, the low battery voltage can also cause problems with the injectors. They may not open easily or properly, which can cause lean misfire or hard starting. The low voltage of the battery can actually be the cause of some drivability issues and starting problems.
The battery of my GMC R2500 has been fully discharged. What would happen if the battery is run down?
The lead plates in the car battery can be coated with a layer of sulfate when the battery is run down or fully discharged. As a result, the battery may not store power easily. Because of the changes on the lead plates, it may not recharge fully or instantly due to resistance caused by the sulfate layer. This will reduce the battery life, which on average can be 4 or 5 years or even shorter if the vehicle is driven in extremely hot locations. Low temperatures or cold weather may also reduce the battery's power output. Under this condition, higher amps will be required in order to crank a cold engine.