I agree, its such a simple thing to forget (if you need to replace the filter, they are cheap enough from Lexus), a good clean out should help. Its a very fine gauze mesh and you may find it chocked with old oil, this is something that is missed with a service (as its not normally something that people think of) - I used 'gunk' and a small screwdriver to drag out the rubbish, but don't be heavy handed or you'll break it. Its so small that once you've taken the bolt out you will need small pliers just to pull it out (as not attached to bolt, as you'd think). Difference it made was noticeable and even my local mechanic now looks for it when he services all types of cars.