Each lifter has a very minimal amount of leakage and will compress under pressure albeit relatively slowly. In normal operation oil pressure pushes the lifter out of the rocker until it contacts the valve. When the valve opens it happens so quickly that the lifter doesn't have time to bleed down thus it acts as if it were a solid body. Any small loss of oil out of the lifter is refilled before the next valve actuation.

I wouldn't worry terribly about filling each lifter full before installation. That job belongs to the oil pump. Just get as much in them as you can without making them too stiff to install without difficulty.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff