Major oil leak, mysteriously fine next moring-???
 

I took my 98 XK8 down to our place on the Ga. coast, about a 5 hour run. I try to stay off interstates, as the ride through South Ga is pleasant. About an hour from Jekyll Island, and 10 miles from Jesup (middle of nowhere)
my low oil pressure light comes on. I immediately pull over to a church, in the dark. No reading on dip stick. As a knowledgeable Jag owner, I keep 5-6 quarts of oil in the boot at all times. Takes 3 quarts. I am very aware of my minor Rear Main seal leak, which is typically about a quart every other fill up, so this baffles me. I proceed into sleepy Jesup and as I pull into service station, light comes on again. I look under and there is a 1/8" steam of oil running down off bell housing. I put another 3 quarts in and just barely reads on stick. I back away to check further and a line of oil on ground. As its 11 pm, I check into a hotel in town about 1 mile away. The next morning I'm fully expecting a huge oil puddle under the car, but to my great surprise, NOTHING! I top her off and find a mechanic to put up on lift. We can find no major leaks, but entire under-body is evidence to contrary of previous nights leaking. I drive in 15 minute spurts around the shop continuously checking the oil status. After an hour still no leaking. At this point I continue on to the Condo, drive all week, and then return home and never again did this major leak occur nor since in my few short drives.
Anyone have any clue what could have happened after 4 hours of driving, and then fixed itself overnight??

Maybe rear main seal worked loose but could be Rear oil gallery plugs
 

You may have gotten lucky. Obviously you had a major leak, where you know you’ve got a rear main seal leak. This could have become dislodged, leaked, and then worked back into shape.

Hope it’s not nothing new like rear gallery plugs.

Maybe it’s a sign for you to get the engine and tranny out and do the job(s)

Very strange, but I agree the only choice is to drop the transmission replace the main seal and check the plug.

First, I'd go for checking the oil filter and oil cooler lines if applicable. Be a shame to drop the transmission to fix the seal and find the filter was loose.

Oil filter check
 

+1 to Truck Graphics suggestion on checking the oil filter.

One of the common faults on home DIY oil changes is the oil filter.

https://cimg7.ibsrv.net/gimg/www.jag...a3a563a91.jpeg
Sometimes when you remove it, the old sealing ring stays stuck in the car. If you don’t notice it, then you’re screwing the new filter in with both rings in place so it’ll leak.