No leaks so far. I am guessing that the new Dorman orings are bigger than the Dorman old ones. However, if they leak down the road I will take the advice and pop on the AC Delco ones...

I have not had to order these myself since the Fel-PRO ones I used have not leaked, but if ever needed, these should be the correct Delco O-Rings.
ACDELCO Part # 1534066

My 2000 Leabre has 2 elbows...they are different sizes so I assume they take 2 different orings ??

On the upper elbow the two are the same, as well as one of the o-rings on the lower one, there is one o-ring on the lower that is bigger than the rest, that one does not seem to cause any leaks, the dorman o-ring actually worked better than the one that came with the Fel-PRO kit, the one that came with the Fel-PRO LIM Kit seemed a bit smaller to me.
I'm not sure which Delco o-ring to get for replacing that larger one, I'll try to find it for you just in case you do not want to use the Dorman one.

However, that would be great to have its part number (just in case).

This is a great forum.

Leave it to William, he'* like a Bloodhound when it comes to looking for parts.

He he, thanks Mike, I do know how to sniff em out.

I did a little research, and I am pretty sure I found the right replacement o-ring from Nalley Buick Parts for $2.18 plus shipping.
This link should work, if not let me know.
ENGINE ASM-3.8L V6 PART 3 FRONT COVER AND COOLING. Fits: 2000 Buick Lesabre CUSTOM | Nalley Buick GMC Brunswick

PARTS
Larger O-Ring is listed in the diagram as #343 GM 24502846


The other O-Ring on the lower coolant elbow is listed on the diagram as #343 , it is listed as GM 25537068, and they have another number listed which is 1534066, which just happens to be the part number I posted for the upper coolant elbow o-rings, so it appears they are the same after all.
I cross referenced the GM 25537068 upper coolant elbow o-rings to see if they were also the Delco part# 1534066 like I thought, and sure enough they are.
There was no alternate number listed for the larger o-ring.

See, the bloodhound prevails!

Go get yourself a Scooby Snack Will!

William - you are awesome. Thanks!