Rear main seal leak?
 

Noticed that I've been losing oil but there is no drip or oil spots where the 4runner is parked. It's a 1990 manual with 350,000km. After looking underneath I noticed that there was a lot of oil on the bell housing by the slave cylinder. I checked the clutch fluid and it's at the max level so I'm sure it's not that. It looks like the oil is coming out of the hole where the slave cylinder rod (for lack of a better term) goes into the bell housing.Could this be the rear main seal that's leaking then?? Or could it be coming from the transmission? It doesn't have the nasty gear oil smell so I'm leaning more towards it being the rear main seal, especially for how old this vehicle is. I will check the gear oil level once I can get it parked somewhere on level ground first.

It also seems that it doesn't leak unless I'm doing a long highway trip. After I cleaned up the area I went for a ride, maybe 40 minutes just around town going at a top speed of maybe 70 km/hr. When I came back home it was clean. Just today I took it on the highway for at least 40 minutes going around 100km/hr and now the bell housing is all dirty with oil again.