Oil in Bellhousing 91 GMC Sierra

[WilliamH4]

I'm currently replacing the clutch (clutch disc, pressure plate, release bearing, etc.) in my 91 GMC Sierra 4.3 liter 5 speed manual. When I dropped the transmission, there was oil inside the bellhousing. I'm not sure if it's engine oil, gear oil or both. I decided I should replace the rear main seal (seal at the engine block and input shaft), the seal retainer gasket and oil pan gasket. What seal, if any, would I replace that might have allowed gear oil to leak into the bellhousing? I think I have everything covered that might come from the engine to the bellhousing, but I'm not sure what to replace from the gear box to the bellhousing.

[mjm-1957]

Make sure the back of the intake manifold isn't leaking sending oil into the bellhousing.