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LT's are notorious for leaking at the back of the manifold, Check that first before you go pulling out the trans, I had a leak and swore it was the rear main, but it turned out to be in the back of the manifold, the oil just rolled down and settled at the rear main and it looked just like it was coming out of there.
Do you think I 'll be able to tell for sure which it is when I remove the transmission?
They can be tricky to find....might even take some dye in the oil and using a black light to track it's source.
I've got a small weep in the same location as you that I can't track down. Shows up at the same spot as yours. Never weeps at idle....only after road trips. Never leaves oil on the garage floor. I replaced the rear main and resealed a couple of areas only to not solve it. I pinned a blue scott's shop towel between the inspection plate and trans to see how much it is really leaking. It had a nickel sized area that showed up on only first layer of the towel after a week of driving it daily...didn't even go into the second layer....so it is a minimal leak...thank God....I hate oil leaks.
Just as the others have mentioned, check the back of the intake manifold, rear main seal, the oil pan seal, and make sure your oil pan bolts have not worked loose. I had that happen one time causing a nice leak too....that did show up on the garage floor.
Maybe even check the base of the oil dip stick tube. I doubt it's that, but there is the potential of a leak.
If you do chance the rear main, be sure not to scratch it.
I would clean the whole engine with a rag, no water, and get it completely dry, then drive it and follow the new trail. Could be the back of the intake, could be the valve cover gaskets running down the heads, could be the rms.
Intake leaks are usually to the sides of the engine and not dripping from the trans. As stated, feel the back of the intake. Could also be leaking from both places. If the back of the intake is dry, next place to look would be the RMS. If you see any accumulation around the circumference of the RMS, replace it. Use a rubber seal and not teflon, IMO. Rubber is much easier to install. If the back of the intake is wet, reseal the intake. Should be how to's all over the place. Use brake clean and dry everything off so there are no residuals, drive around and see if you see a leak.
yes it can be the RMS but understand oil leaks from above migrate down to the lowest point which can make them appear to be coming from that point vs somewhere above.
The rear of intake manifold is prone to leaking, dripping down between bellhousing and block collecting on the crank like in your pic
with that said a RMS leak will look exactly like what you have. Do as suggested and first confirm the leak is not from rear intake manifold, oil sender fitting, valve covers and even oil filter adapter first
I chased what I thought was a RMS or pan gasket leak, replaced both (huge PIA) only to find it still leaked and looked like your leak....got oil dye kit and after pulling trans again could see the leak was from the gasket between the RMS adapter and block. Had to drop pan again, replace that seal and put everything back together
so do what you can first to define where leak is as pulling the tranny & clutch is not a small job. If you are convinced it is RMS you will see a trail from rim of seal going down. dye helps identify that trail in neon green with a blacklight
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Leak at rear of 96 Lt1 
