RE: Replacing Seals in 3.0 engine
i don't know anything about your 98, but i had an 88 with a 3.0 v6 a long time ago. mine was not too bad to work on. the front of the engine was on the passenger side, and by jacking it up and removing front tire and inner fender liner, you can get at everything pretty good. i once put on a water pump and timing belt. just strip off belts, ac, etc, etc and eventually you get down to a bare motor.
the first thing you need to do is locate a factory service manual (first choice) or haynes manual (second choice) and read up on the procedure. if you have oil leaks, one of them is likely the front crank seal. i don't know - but once the front of the motor is stripped, it should be easy and cheap to replace it. the rear main crank seal may be a different story.
valve seals can also be replaced. but, when they leak, it causes excessive blue smoke in the exhaust, not an external leak. i once paid a guy to do this, and how he did it was strip the top of the motor and remove valve covers and spark plugs. one cylinder at a time, he lowered the piston, fed in cotton string through the spark plug hole, and then rotated piston up. this pinned the valve. then he removed the valve sping holder, replaced the rubber seal, and reinstalled the spring. very clever. i think he charged me about $250 or so.
there are not too many places that an engine can leak oil. valve covers, oil pan. front and rear main seal. cam seals ?? intake ?? head gasket. you can probably easily fix anything except the rear main. i don't know about it - if its a 2 piece seal, you can do it w/o engine or trans removal. if its a 1 piece seal, then you got to pull, or drop, one of them.