All of the older cars, back in the early 50's had a solid lifter, which as the name implies, was a solid peice of metal that rode on the cam lobe. The solid lifter would push against the end of a pushrod which pushed against a rocker which pushed against the end of the valve. With all of these peices of metal in the valve train, as the engine heated up and the metal expanded, the valve opening and closing would be affected by temperature. If the "valve lash" was set properly for an engine running at normal operating temperature, then when the engine was started cold, all the peices in the valve train would shrink a bit (shorter) from the temperature, then the valve lash would change and everything would rattle, because of the increased gaps between all of the components. Then someone found that if you built a 2 peice lifter, like a small hydraulic cylinder, then the engine oil pressure would (pump up) the lifter, making the lifter extend lengthwise untill the "slop" between all of the valve train components was gone. This worked great, because as the valvetrain expanded or contracted with temperature changes, the hydraulic lifter would always keep the components firmly seated against one and other, thus eliminating the rattles and constant frigging around adjusting the valve lash. If you have an old car designed for solid lifters, you can not put hydraulic lifters in it, however if you have a newer engine designed for hydraulic lifters, you can put solid lifters in it. Some people who build racing engines will use a solid lifter, because at really high rpm the hydraulic lifters don't have time to get fully pumped up with oil, and so consequently the valve lash and timing suffer at high engine rpm. The amount of oil that a hydraulic lifter uses to pump it up is very small, because in reality, they only expand about 1/16" in length.