Bingowings said:

Droids are pretty much like washing machines in the Star Wars universe.

Owen's initial rejection of Threepio could be down to having one for a bit and not being that impressed (and then buying him anyway because his wife keeps using Luke as a proxy nag-o-phone).

Obi-Wan's droid fib (and yes he is prone to lying through his teeth thanks to George's many retcons) doesn't make any sense at all.

What does Kenobi hope to gain by either not telling the truth or perhaps being an uber-pedant?

OBI-WAN said :

I don't seem to remember ever owning a droid...very interesting. I was on first name terms with a few, including one that looks exactly like this one...funny little fella we used to call him Artoo but I never actually owned one.

Someone once said that the burden of meshing the two trilogies together via fan editing should be placed on the PT, but I've thought that such would be far too difficult in some cases. However, just this morning I actually thought of a simple idea to correct this little mistake. A little voice splicing and cutting to Luke's face could resultin a line such as, "I seem to remember a droid like this one...very interesting." I don't know exactly where to get the "like this one" part, but something along these lines might be doable, simply showing that he vaguely remembers R2, but that 20 years have obscured his memory and he can't be sure.