[JFP in PA]

You are correct, the first cars (the 97 2.5L) had no line on top of the cooler, and in fact used an adaptor plate between the cooler and the engine case to mount the cooler (using even more orings). These cars have some issues with air entrapment significantly reducing the heat transfer capacity of the cooler, leading to oil breakdown and bearing issues. In 98, the return line on the top of the cooler appeared to address the air problem, along with another hose coming up from the oil pump housing, I assume for the same reasons (I have not heard another explanation). The adaptor plate under the cooler also disappeared during the 98-model year when the engine cases changed to allow direct mount of the cooler. Sometime during the transition from the 2.5L to the 2.7L engine, the line from the oil pump housing also disappeared (not exactly sure when as Porsche has often blurred these transition points) and we were left with the adaptor-less cooler mount and the single line from the cooler to the surge tank you commonly see today.

Worst part of all this is that parts for the early cars are nearly non-existent; the only replacement coolers listed are direct mount style with the surge line, none of the adaptor mount coolers without the surge line can be easily obtained, or any cases with the line coming up from the oil pump.

Just a note in passing, you cannot do one of these larger oil cooler updates on the 987 series base engine, the changed the layout so there is not enough room for it to clear. I’m not sure if that applies to all 987 base engines, but the ones I have looked at all seem to be that way.