About to do a heater core replacement on my '96 960. Are there any drawbacks to using the Black reservoir cap over the Green cap that comes on the 960? Since I've ordered the Nissens heater core with plastic end tanks I want to get as much life as possible doing this huge job. Or is better just using the Green cap the 960 comes with.

My '91 745 uses the same heater core as the 960 according to the fitment list. The car is 5 years older and as far as I know still on the original core and using the Green cap. Have had the 745 going on 10 years and still doing fine. That may change now that I've mentioned it. The 960 have had around 5 years with an engine replacement last year.
745 has 204K
960 has 202K and engine 10K less at 192K.
Thanks Dave

Black cap 75 kPa (about 10.5 psig); green cap 150 kPa (about 21 psig). Downside - not as much boilover protection with black. Upside - hoses and other components dealing with lower pressures. I run a black cap on my LS powered 242. An aside - when the car was 5.0L powered I moved to green cap (mine came OEM with black cap). On the dyno it blew a heater hose off one of the firewall connectors. I attributed that to twice the pressure it was designed for. Bypassed heater core for ride home. Expanded the “lip” on all heater hose connections. Put black cap back on.
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82 242-6.2L; '17 Mazda3; '16 Crosstrek