I've got multiple DT's and RT's (as well as others) from the early 70's.  The clutch sticks on the lowest mileage one I have, a '73 DT3.  None of my other bikes for all these years have ever done it. In the past I've been able to crank it and warm it up some in neutral, roll it down the driveway and pop it in gear and then blip the throttle several times with the clutch lever in to break it loose.  Problem is it won't break loose anymore.  So last night I pulled the right side cover and the clutch springs and pressure plate.  The plates with the friction material were stuck to the steel plates, but not severely, and came apart easily by hand.  Everything was oily and looked new inside.  So the questions:  1.  WHY is it sticking in the first place.............. did somebody use the wrong oil?  2.  The plates look perfect.  Is there anything I could soak 'em in to remove whatever is making 'em stick or have I gotta spend almost $100 for new plates?  3.  GM posi traction rears have an additive added to the oil so the clutches don't get sticky making the rear kinda jerky going around corners.  Would it be a good idea to add a little bit of that to the bottom end oil in the Yamaha?

My '71 DT1 plates stick sometimes, and sometimes not. Colder temps make it a little more prevalent, and sometimes how long between rides. But also sometimes not...

Me, I would hit the steels with some 600 grit sandpaper to scuff them up a bit, and change the oil using a legitimate motorcycle transmission oil, say something from Maxima, 10W-40. I wouldn't change the frictions at this point. Definitely do not use an additive for posi diff's as that additive would probably stop the sticking but introduce slipping under hard acceleration. 

The last time i had this problem was on one of the Gas Gas trials bikes i've had.
I stripped the clutch and cleaned the frictions off with a cloth . I left them on a piece of thin ply overnight then flipped them over the day after. Later i washed them in acetone for a minute and dried them off and left them spread out again over night. Re soaked them in gearbox oil .Threw them back in and problem solved .
I placed them on the wood so the oil leached out and soaked into the wood . I only use mineral oils in the DTs . I read in some Castrol literature that if you use incorrect oils in gear box's , an acid can be formed due to pressure, heat and foaming . In some cases (they state) this can cause corrosion or attack certain resins, rubbers, and other materials used in different gearbox application.
So, to high light how aggressive additives can be in oils the semi synthetic 2t , Power one, oil shrinks oil pipe and makes it brittle when i've used it on these bikes . So i tend to take their research seriously. I also see some gearbox oils emitting vapour (Not for bikes) i
so the chemicals in them are quite harsh in some cases. Maybe that's the case with your clutch plates.
I agree with your decision to use designated oils.

Pity plates are so expensive for your DT as that'd sort it along with cleaning up the steels as well. My LS3 is 'so' stuck i'm just gonna have to go in & break the spell as don't think its ever been off. Could leave the lever tied back between rides but shouldn't have to ideally. On bikes i've had that get a bit that way i've hooked top gear sitting still & smoked up the clutch--well not quite but given it a good slipping & always seemed to help, for a while. Not recommending it though. Used to do that with one old car to fix shuddering. Worked for a while. Slipped it until smoke came up through the floor boards haha.
Looked in the oil filler hole on the ls3 the other day thinking--should i-- na better not. Could see the plates & want to get a flat blade screw driver to tweak them but i'm sure they'll break.

So is any oil available from the chain auto parts stores that would be appropriate?  I don't have a motorcycle shop closer than an hour away.
 

You could get it shipped to your house.
www.amazon.com/gp/product/B0723C626F/ref..._title?ie=UTF8&psc=1
What did you decide on??

Went with the Castrol that Deet recommended in an older discussion. (non synthetic 4T) So far so good. Have ridden it several times in the last week with no issues.