Correction fluids have changed since the 1980s, but not by much. The chemical composition of correction fluid is organic solvent, so that when left unused and exposed to the air, it thickens and can become solidified. This is how you thin it out.
Use a solvent from the manufacturer of your correction fluid to thin it, if it is not too costly. Each manufacturer has its own brand of thinning solvent.
Aim the open top of the solvent thinner from the manufacturer toward the open top of the correction fluid bottle. If you don't have a steady hand, you may want to use an eyedropper.
Target your linen closet and look for nail polish remover if you elected not to purchase thinner from the manufacturer.
Drip the nail polish remover into the correction fluid bottle with a steady hand or eye dropper just like you would with the manufacturer's thinner. This is flammable and has strong fumes. Vent your work area when using any type of solvent.
Shake the bottle of correction fluid with the cap screwed on securely.
Treat a piece of paper with the thinned-out correction fluid to see if the desired consistency has been obtained. If not, then add more thinner until the job is complete.
Put down newspaper or paper towels to prevent damage to surfaces and clothing.
Do not smoke when using thinners.