This is not the sending unit...its the oil thermostat above the sending unit....the two springs that control when the oil cooler is open fatigue over time..... Replace both springs and clean the center unit....so you will need the two springs (one short one long) and the seal for the sending unit....its a bit of a PITA to do, since you need a special short wrench to get it off....I took the whole thing off as a unit from the block, then separate the sending unit if you need to later
The sender unit is on pretty tight and requires an adjustable wrench or a very thin one and I think a 27mm to counter hold.
more pictures, some originate from Tony Harkin, some from Karel Koelman
the heavy spring is the max pressure blow off.....the light spring controls the
bypass port so when the oil in the cooler is too thick the bypass opens to keep
oil flowing to the engine even if the thermostat has closed off. only needs
about one bar of pressure differential. The max pressure blows of at about 9 bar
dumps oil back into the sump
So I guess the thing other than physical clearances within the engine (and cam journal pins) that is controlling total pressure at certain RPMS is the thermostat spring and plunger and the oil viscosity.
no thermostat simply shuts off the recirculation forcing oil to pass into the cooler. The bypass light spring lets it pass if flow is too low (thick cold oil in cooler) . The heavy spring on the blow off sets the maximum pressure about 8-9 Bar. Any pressure less than that is simply all the pump makes given all the leaks that it must feed at any given RPM...