70s and 80s are definitely more recent. I also consider 52 and up "modern" LSATs because they have comparative passages, so I'd use those for full PTs. Anything below 52 I'd categorize as older (but still highly valuable) drilling material.
@WickedLost Perhaps the questions you're looking at fall under a third method of reasoning category? There are three main ones on this test: conditional reasoning, causal reasoning, and comparative reasoning.
One way to look would be going into RC for problem sets, go to the later tests and look at what kind they are. If it says "co" that means it's a comparative passage.