One of the oil sump studs is longer, it protrudes up through a tab on the oil pickup and has a flatwasher and shakeproof nut to hold it down. Almost impossible to reach that nut on an assembled engine without cutting and bending the edge of the pickup out of the way (yecch!)
You can get 1/12 of a turn on it with a box-end 10mm wrench after grinding away some of the wrench's width right next to the box so it clears the tab. I also heated & bent mine about 45° to make it easier to fish up in there (see photo below)
Double-nutting the stud on the outside so it can be run in or out through the case & nut may save some time.
Most sumps (except Gene Berg's) require that you install longer studs; usually they're included but they may not give you one that's long enough for the "hold-down" location in which case you'll need to make your own. Installing it without losing the nut & washer in the sump is a little tricky. Put a piece of electrical tape loosely across the wrench, reach it up in there and pull down to trap the nut. Have the new longer stud at the ready, already double-nutted. Double-nut the old stud and wind it out, keeping pressure on the shakeproof nut to hold it and the washer in place until you engage them with the new stud. If you do manage to lose the nut & washer you'll need to fish them out with a skinny magnet, so it's best to do this part with the engine right-side-up and perhaps tilted slightly so they can't get too far away.
I prefer to use shakeproof nuts on all six studs to hold the sump on (plastic ones work well enough, they don't have to be the metal spring-type which is preferred for the one inside). Alternatively, you can put a dollop of RTV over/around each one...but you'll need to wait some time, overnight's best, before adding oil.