I'd bet about 100% its your tubes/seals. I've done mine twice in 132K miles. Its not a hard job but does take a bit of time with raising the car, pulling tires and coil packs.
Might want to do plugs while you're in there but not necessary if you think yours are still good.
could be power steering fluid - the ps pump in on driver side and the overflow dumps onto the engine block. could also just be bits of tire getting onto the hot exhaust immediately behind it. could be cv boot on that side throwing a bit of grease.