I've been restoring a 1974 DT 175. Put a new Top end in it and cranked it up and it was obviously burning crankcase oil.
pulled the side cover off and put a new OEM Right crank seal in...very careful with the orientation, put it all back together and it still burns oil.
I did not replace the little "o" ring on the shaft...but it felt tight and looked to be in good condition.
Any ideas? Worst would be a crack in the case. I've not checked the breather to make sure that its patent. Take it apart again and replace that tiny "o" ring?
  Thanks

Put the "o"ring in.....
It may not seem like it needs it but those Japanese engineers were pretty good at building/designing these engines. They wouldn't have put it in there unless it needs it.

Are you running premix and the autolube with the new top end also have you rode your bike to see if it would clear out and checked the oil level in the transmission if you can post a video. Yes it could be the O ring

More likely it's the oil soaked into the exhaust packing along the walls that's smouldering when hot. Burn the complete exhaust out or go for a 50 mile ride at speed.
My last one like that was a Yamy AG100G 1980 which i roadraced once for a laugh. It was an ex farm bike so system was very' oil soaked. Smoked for about 5 laps on full song before it cleared & never looked back.

My only question at this point are the exact details of how it has been determined that it's "obviously burning crankcase oil"?  (I think you meant "transmission oil" as the crankcase oil mixes with and is burned with the fuel in these engines)

If you haven't ridden it much or at all, that's the first step...  especially if it's been sitting unused for a few years.  Or if the seal you replaced was leaking.  As RT325 said, it can take quite a bit of running under load to clear up residual oil. 

(I also wonder what you meant by being "very careful with the orientation" as an OEM right side seal on that model can pretty much go in either way.  Both left and right seals have the same OD...  left side original seal does have an orientation being single lip...  you'd really have to force the sleeve into the seal if a left side seal was accidentally installed in the right side...  I've seen that mixup attempted though)

Thanks

Thanks...running Autolube

Thanks...burned the exhaust out with a torch before i put it on. Ran it around the block a few time.

Thanks

Check the main bearings for radial play. That can do it.