Old engine logs are defined as the logbooks prior to the a major overhaul or remanufacture of the same engine, or the logbooks for an engine that is replaced and no longer on the aircraft.
When an engine is replaced or remanufactured, it is common to start with a new set of engine logbooks. When it is major overhauled, you may not see a new set of logbooks, and instead they simply make an entry continuing on from the last engine entry in the engine logbook. Either way, the information prior any of those three events can be important if there was any commingling of airframe data into the engine logbook.
This happens when mechanics accidentally include airframe information in an engine logbook entry. That information is usually meant to reside in the airframe logbook as a permanent part of the aircraft records. Shown above is a real engine logbook entry example where this happened. We redacted some sensitive information, but notice that the green box (line item 3) pertains to installing an LED Belly Beacon onto the airframe. If you were to toss these engine logbooks out without at least scanning them first, you would never know when that beacon was installed.
Another reason is for AD (Airworthiness Directives) history. While all applicable ADs are typically complied with at the time of an overhaul, remanufacture, or manufacture, there are rare instances in which being able to find AD history prior to that event can be important.
It can never hurt to scan old engine logbooks. For all of these reasons, we advise clients to include their old engine and even old propeller logbooks in their logbook shipments to us.