A few weeks ago I had changed the oil in my '90 4-runner. I noticed some spotting underneath the right side of the engine soon afterwards. Nothing much - about a tablespoon.
I had the seals replaced about 25k miles. Finally I took it to the garage. They told me the filter and drainplug where a little loose and the gasket around the pressure switch was also wet but not dripping.
I can't believe I didn't tighten the filter and drainplug down for the first time in 15 years. Must be getting old.