95 EX with B20 block and LS head. LS manual transmission.

Recently replaced oil pan gasket which solved one of the oil leak.
Other leak is hard to pinpoint. Definitely not coming from valve cover. No oil underneath distributor which seems to rule out distributor o-ring and oil sending unit o-ring. No leaks from oil filter gasket, cam gear cover/gasket, crank seal.

I see crusting of oil around the transmission to engine mating surfaces. This includes the top of the transmission too. Fresh oil that leaks above and puddles at speed sensor location. Also see oil on the transmission shift linkage and bushing near the engine.

It's hard to see exactly where it starts, but seems to be very near the thermostat housing. However, there's no oil source at that exact area.
Can a rear main seal leak cause oil to leak in top and sides of transmission, as well as the bottom?

 

Cleaned engine, transmission and shift linkage very well yesterday.
Took it for a highway drive this morning and oil appears on shift linkage again. The rest of the engine is dry otherwise that I can see.

Now I am leaning towards the driver side differential's output shaft seal as the likely source. However, the oil is thinner than gear oil and doesn't smell as strong as gear oil does. The prior owner did all the swaps and may have used 30w oil in the transmission instead of Honda MT fluid.

 

The Honda MTF isn’t gear oil, so it’s not thick and doesn’t really have a smell. It’s more similar to 30w oil, but a bit thinner if it’s the newer stuff.

Make sure the speed sensor is fully seated, that the o-ring is in good shape, and that the mounting ear is not broken or cracked. It’s pretty easy to break if the bolt is installed before the sensor is fully seated or if used to pull the sensor home. This is all much more likely if the speed sensor hole has been used frequently as the gearbox oil fill.

An axle seal spewing oil all the way up on top of the gearbox sounds a bit sus, but I’ve also seen pretty big messes in that area from one. You could always drop a half bottle of UV oil dye in through the speed sensor hole, get the thing really clean, and check it with a black light after a short drive.

 

Thanks for the reply. I watched a video earlier today showing an MTF drain and fill of an Acura LS using the Honda MTF fluid. As you mentioned the fluid looks thin and clear. I've done an MTF drain and fill on a 6th gen stock several years ago and aware to always looses the fill plug first prior to drain plug removal. I have a bag of Honda 14mm crush washers that should work for the two plug bolts. It's been about 35k miles since the previous owner did the swaps, so I figure I might as well do a drain and fill for regular maintenance and peace of mind.
I've encountered leaking output seals in the past and they generally get messy real quick. I figure I'll spend $10 for an Honda genuine output seal to have it on hand if needed. Great idea to add the dye and I have a bottle of yellow dye when I checked for an HVAC leak recently. Can that dye be used for oil?

I'm keeping an eye on the speed sensor gasket. I cleaned the that area off really well and after the drive this morning it looks shiny/wet, but no noticeable leak from it.

 

They make UV dye for engine and hydraulic systems.

 

Just a note, having the same issue with mine. Didn't know axle seals aren't supposed to be pushed all the way in; rather they are to sit flush with the trans. Previous owner could have made the same mistake I did, and it might be leaking from that seal instead of the rear main.

 

The inner cv joint is flush with the differential, but that seal can wear over time and leak. The dude I bought it from is meticulous and did several swaps prior to this one. I've owned the car for six years and this leak started a couple months ago.