I don't think this is a really big problem but it can be very expensive if it occures to you. I have a buddy who races a wr450 in the WORCS series that has had this problem twice since he's owned the bike (new). After the second incident I asked him if he was bleeding the air by loosening the small 8mm screw next to the banjo fitting on the cylinder head (I believe his reply was 'what screw?'). If you change your own oil in your yzf/wrf 250/400/426/450 the service manual explaines how to bleed air from the lubrication system after changing the oil filter. When you change the oil filter there will be air trapped inside the oil filter housing. If don't do this it takes a very long time for the air to be purged through the oil lines and passages. If you don't want to use the bleeder screw or crack the banjo bolt then lay the bike on its left side and fill the oil filter cavity with CLEAN oil before fitting the new filter and cover.