Loss of oil pressure

The FILL cap? No not unless its a sealed system. I don't know of any production cars that use a sealed syst. But replace it just the same, you don't whant dirt getting into the engine. How many miles what engine and what wieght oil did you use?

After checking the oil and finding it a 1/2 quart low I added about a pint of 10-30 and a pint of additive (you know the thick stuff)
I then drove about 3/4 of a mile to a store without a problem. On the way home I noticed the engine dogging down pretty good and then a few moments later the oil light came on. Right away I realized that I forgot to put the filler cap back on and I stopped to put it back on. The light stayed on for a bit then went off. The engine also began running fine.I drove it to work today without any apparent problems.I'm thinking it was either the cap or maybe the additive clogging something up for a bit. Correct? The car is an 88 Mazada 626. What do think about any potential damage? Should I get the additive back out and clean the screen? Thanks much for your help.

Oh yeah, the motor has approx. 154,000.

Yes I checked it right away and it was fine. No oil on the engine either.