Yesterday I found some water under the car and was not sure if it was through the garage wall or from the car. The radiator is topped indirectly from a plastic bowl with a plastic screw cap to the left of the radiator normaly you can see the water level through this plastic bowl but it is now grubby with age and difficult to see through. I decided to take the plastic cap off to check the water level but it was very tight and I had a struggle to unscrew it all the way out. It is also going to be very tight to screw all the way back in (its not cross threaded) and I am afraid I will not tighten it down enough.The cap is probably tight because its not been unscrewed many time as the car has only done 20,00 miles. Would it be safe to spray a few drops of oil or spray silicone lubricant or wd 40 on the plastic cap thread to ease it or would these cause damage.

Thanks