mrporth writes, "...a MAJOR puddle of oil forming."
Go out there to your puddle of oil. Squat down, run an index finger through this oil. Take a lick of this oil. If this oil tastes really bad, might be trans oil, might be engine oil. What you have learned is tasting automotive oil leaves a bad taste in your mouth and you have learned to never do this again.
Determine where this oil is emerging, with certainty, absolute certainty. If coming from your flywheel cover, this can only be trans oil or motor oil. I will place my bet on motor oil.
Transmission oil will be very thick and very slippery, along with having a strong pungent odor, an odd odor. Transmission oil will stain a white cloth or white paper, a very dark brown with a ruddy red hue around the edges of the stain, after well absorbed.
Motor oil will be thin, spreads quickly, soaks in quickly. Motor oil usually has very little odor, at worst, a burnt smell. Motor oil will stain a white cloth or white paper a black color right out to the stain edges. Only exception on motor oil color is when an engine has been severely overheated many times which cooks the motor oil to a deep brown tinge, but mostly black. This cooked motor oil will have a very distinct "burnt" smell.
Here is a quick test to verify you are dealing with a motor oil leak. Fetch a piece of white printer paper, don't leave greasy fingerprints on anything, your wife will yell at you. Take off your oil fill cap, swipe your finger inside your engine or inside the oil fill cap. Dribble a drop on your white paper. Swipe your finger in your puddle of oil on the ground. Dribble a drop on your white paper near your oil cap drop. If those drops soak in at the same rate, have the same color and thickness, smell the same or no smell at all, you have a rear main leak. Otherwise your leak is from your transmission.
Another verification is to drop off your clutch dust cover. If an engine oil leak, your flywheel will be wettest with oil on the engine side, noticeably less wet on the transmission side.
None of this really matters. If a rear main oil leak, you need to rebuild your engine, might as well rebuild your transmission at the same time. If a transmission leak, you need to rebuild your transmission, might as well rebuild your engine at the same time. Worn out engine - worn out transmission. Do 'em both and replace your clutch assembly to boot.
Many claim you can insert a new rear main seal into your engine without pulling your engine. Don't bother, this never works. In time, your leak will be back and probably worse. This entails lifting your engine a bit so you can remove the oil pan and partially drop your crankshaft. This is frustrating. Typically your transmission will hit the floorboards before you can lift your engine high enough to drop the oil pan.
No matter, even if your oil pan comes off without engine lifting, you will have new leaks at your timing gear cover and elsewhere. This is a "half-[censored]" job to attempt slide a new rear main seal around your rear crank, in place. This method will have you cussing before you remove your oil pan; filthy frustrating job which almost always fails in the long run.
I will collect my winning bet on a rear main seal leak, now. Reason for this is an original type rear main seal has a felt boot inside. This felt hardens in time, shrinks a bit, wallah, a leak! Similar problem with a neoprene rubber type seal. The rubber becomes distorted (enlarged hole) then hardens and leaks. New neoprene type seals last a lot longer, though.
A transmission has either a sealed front bearing or a specialized seal, sometimes screw like machining on an input shaft to push oil back into a trans. A transmission, worst case, will only leak a few drops per day; no big puddle. Exception is if the front main shaft carrier bearing cover, a cast iron cover with a tube sticking out, has become loose. You will know, though, because your transmission takes to growling and eventually blows because of too much end play inside. Your throwout bearing will also rattle like crazy. This almost never happens, very rare.
My winning bet is on an engine rear main seal leak, if this oil is coming from your clutch dust cover. Not too many other places engine oil will leak save for your valve cover or engine side cover, neither cause a serious oil leak from your dust cover.
Well, almost. Look at the back of your valve cover, close to the firewall, make sure oil is not leaking out then working its way into the bellhousing through a small space.
You are lucky. Small block V-8 Chevy engines, whether a two piece rear main seal or a one piece, begin leaking oil within minutes of being driven off the factory assembly line.
Best advice I can give you, mrporth, is to start making plans to rebuild your engine, clutch, trans and driveline. This is the "best" method to cure a rear main seal leak; do it all, do it right. You will sleep easy and your wife won't be yelling at you for leaving greasy fingerprints on her computer printer.
Billyray
