Precautions for powering up an old oscilloscope?
Usually the most problematic parts in old (but not ancient) electronic equipment are electrolytic capacitors with liquid dielectric.

The most common problems are:


Some caps dried up (electrolyte dried up).
Some of the dielectric oxide reduced.


In the first case the damage is permanent (unless you change those caps) and the result could be reduced capacitance values (even drastically). This could lead to erratic behavior and even short circuits as a consequence. Usually the problem is apparent as soon as you switch on the power: a fuse could blow up immediately or in short time. If the apparatus is not fused, that could be a problem. 

The second problem is less severe. If charged at reduced voltages the oxide layer will form back in minutes and everything will be fine. Of course you would need to power the device with a reduced supply. 

But you could try a bit riskier operation, which often works: power the equipment up for a short time (5 seconds), if it doesn't break because of the first problem, it usually can bear its full voltage for such a short time. Then power down and leave it alone for half an hour. Then repeat, now keeping the power on for 10s, then power down and let it rest for half an hour again. Repeat again, ever increasing the interval you keep the power on (1min, 2min, 5min, 10min). Always keep an eye on the thing while it is powered up, looking for magic smoke, funny sounds or strange smells (and shut down the thing if it happens). When you reach a point where the device can survive half an hour of continuous operation probably its caps, if not damaged because of the dry-up, will be regenerated.

Note that 8 year storage is not too bad, if the device was stored in a dry, cool place, with no temperature extremes or other atmospheric "danger factors" (salt from seaside air, for example). I expect it to be in good shape since it is professional equipment from a reputable manufacturer, and not some el-cheapo consumer electronic gadget.