Well, it seems the seller has some problems in measuring the current consumption of the second Pentax spotmeter, so I doubt I will get that information.
In the meantime, I found something interesting on web:
1) The silver oxide batteries produced by different manufacturers have different discharge characteristics (see attached file). The Pentax spotmeter (as measured on mine) uses about 20 mA of current. With that drain, the battery voltage for some silver oxide batteries can drop to 5.6V within 1 hour of continuous use. Similar happens with 2.5mA of continuous use, just later. The voltage then drops very slowly to 5.2V. The voltage on batteries I use dropped to 5.7V in less than 1 hour of (intermittent) use.
However, another manufacturer lists data according to which the batteries keep their voltage above 6 V until discharged (last figure in the attached file, data given for 2 mA of continuous use). This makes me conclude that it is best to buy the batteries made by a manufacturer that specializes in photo equipment (or the one that publicly releases the data on their batteries).
This said, I didn't notice any difference in the reading of my Pentax spotmeter as the battery voltage was dropping to the current 5.7V, as compared to the reading of Minolta F.
2.) There are difference between the Pentax and Minolta spotmeters that should obviously result in different current consumption. Minolta used Liquid Crystal Display (LCD), while Pentax uses a display that contains elements reminiscent of Light Emitting Diodes (LED). The LCD-type displays consume much less power (no light is emitted by LCD's) than the LED-based ones. This also means that the Pentax spotmeter does not need an additional light to illuminate its display when used in low-light conditions. Minolta, on the other hand, does require a small light bulb in these conditions: with light turned on, Minolta consumes about 10mA of current.
I think this settles it all, I should have realized this long time ago: the current drain I measure on my spotmeter must be its normal consumption. Additionally, the difference in the readings between Pentax and Minolta spotmeters of 1 EV seems to be due to a different calibration standard that was used years ago for some of Pentax spotmeters (as discussed on some of the threads on this site). My solution is to rate the film 1 EI faster.