When they say ALL the oil the oil they mean that. If Changing fork seals it is no problem, you have the forks apart totally any way and can flush them with any solvent you like which cuts oil but does not interact with the plastic bits. Gasoline is fine but very dangerous for flash fire if you do not use extreme care and perform the cleaning out doors well away from sparks, fire and the jerry can with the fuel. Dry the parts with an air jet and coat with a film of fork oil before installing. My KLR manual suggests using copious amounts of soapy water ,tehn blow drying. Trouble with water is its slow drying and hard to get out from between layers, may cause rust.
Some fork tubes are ragged and sharp (KLR)at top edge and can nick fork seals so put a single wrap of black plastic electrical tape stretched around top of tube , half on the tube .and coat with fork oil. The seal will easily slide over it onto tube like a shoehorn, no cuts which would require a brand new seal.
If you take the fork apart and let everything drain off the springs and tubes and wipe them with a clean cloth that should be clean enough of oil to re-install. Just flush the fork slider and damper rod to get the sludge out and give the threads on the bolt and damper rod a sprits of contact cleaner, brake cleaner to clean them off to bare metal so that the locking compound will have a good grip.

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93

Poor the old oil out, use copressed air, and use som fresh oil to "flush" and then air again.

In mine you need to fill 574ml (14-15cm from the top, fully compressed, with no springs) og something like that, in each, so you'll end up buying some extra oil anyway, use it.