|The initial oil pressure for a cold engine is normally 50-65 psi pressure. This should appear on the gauge somewhere between 4 to 10 seconds after the engine is running, the exact time depending mostly on how many days it's been since the engine last ran. If the time for the pressure to rise is much longer than this, say 15 seconds or more, you most likely have a problem with the oil pump.
The oil pressure drops as the engine warms up and the oil becomes less viscous. Once the engine, coolant, and oil have all reached normal operating temperature -- after 15 minutes of driving, perhaps -- the pressure will depend on the viscosity of the oil used, and on engine rpm. Using 20W50 in an engine in decent shape you should read about 10 psi per 1000 rpm up to a maximum of 50-60 psi. If you're using 10W40, or the engine is very hot, expect 5 psi less, everywhere. Yes, this does mean that the pressure may be very low at idle, but unless your bearings are completely worn out, or the oil pump is worn out and can no longer produce enough oil flow, you shouldn't have any bearing rattle even then.
The thing to watch for, other than an extended oil pressure rise time, is occasional fluctuations in the pressure when the engine is operating at a constant moderate speed such as 2000 or 3000 rpm. These appear as sudden short-term drops in pressure, typically about 5-10 psi. This is a positive indication that complete failure of the oil pump can be expected to occur within just a few minutes.
Fortunately, with a JH, the average attentive home mechanic should be able to deal with a worn oil pump over the course of a weekend, with no expense above the cost of a few gaskets, a couple of o-rings, and perhaps an oil seal or two. In rare cases, one will also need new oil pump rotor(s) or other parts but this can not be known until things are apart. If a professional mechanic's services must be obtained, figure on paying for 4-8 hours' labor depending on his ability and familiarity with Lotus engines.
On the JH engine, oil can leak from a remarkable number of places and you'll need to find out where the oil is coming from. Though there are a few nasty peculiarities here and there, the fix is generally just a matter of replacing the offending gasket or seal. In any event leaks are a separate issue entirely from low oil pressure.