So I have a Yashica D camera I have shot new film with. But I want to shoot one roll of this film and there was one exposed roll. The film is dated 65 to 67. I was told to take the F stop on stop open for each ten years out of date. I will bracket this to see how it works. Any advice extra? I am going to sell some off and let others play with it or use it as display.
I'd consider doing stand processing on this roll. If you don't develop yourself, now would be an excellent time to start, as labs don't offer stand developing.
Traditional development is, relatively, very precise. That is, traditional development is very sensitive to exposure, temperature and timing. Developing this way usually lasts 7-15 minutes, while agitating the development tank every (half a) minute or so.
Stand development is the opposite to traditional development. With stand development, you use heavily diluted developer and just let the film sit in the tank for (usually) over an hour, without agitating the tank. Because of this, you have much more leeway in exposure. This you will need, because film expired for this long will not behave as you expect it to, even though the 1-stop-per-decade rule is definitely a good one to follow.
A more thorough explanation by Stan:
This technique uses a "develop to exhaustion" concept. Developer in contact with overexposed areas goes as far as it can. When depleted, (out of chemical energy) the processing stops automatically due to inactivity. In the shadow areas, however, there is less developer by-products to inhibit developer depletion so the shadow areas receive continued processing over a longer time. The processing is a means of compensation for exposure latitude. Another similar technique was to clamp developer saturated film between 2 glass sheets so developer could not migrate. A-B developer works this way too.
There is also semi-stand development, which included minimal agitation.
Links for you to read (development in general and stand development:
I can't speak to stand development as I've not done it, so this answer is really about conventional processing.
If you assume that that all the rolls you have are from the same era and have common storage history (neither of these may be true: you can at least check they are from the same era by looking at lot numbers & expiry dates, and if they are there's a reasonable chance they share storage history), then given you have a bunch of them it is reasonable to treat one as sacrificial and use it essentially as a test strip.
To do this you want to make a bunch of photographs of the same scene under the same lighting, but with different exposures. Ideally you would use some kind of test card with strips of various levels of grey on it, but without that you can generally construct a scene which has both some really dark shadows, some really bright white, and a good range of greys in between. The scene should be as two-dimensional as possible so you can use very wide apertures which you may well need. And you need to control the lighting reasonably well: I am lazy and I just take the exposures fairly quickly on a day with fairly even cloud doing basic sanity checks with an incident meter so I can know the light is not changing more than half a stop at most. Put the camera on a tripod so you know it's in the same place each time.
I think your stop-per-decade thing is fairly well-accepted (despite some answers here!), so given this film is about 50 years old you're looking at about 5 stops down from ISO 400, which is ISO 10. I'd guess it will probably be a bit faster than that. So, if you have 12 exposures (6x6 camera) to play with make ones at, say
- ISO 1
- ISO 2
- ISO 5
- ISO 7
- ISO 10 (estimated 1 stop per decade speed)
- ISO 15
- ISO 20
- ISO 30
- ISO 40
- ISO 80
- ISO 160
- ISO 320 (box speed pretty much)
The exact ISO speeds you choose are up to you: I've picked a roughly power-of-two sequence (ie one stop per step) but added some extras around 10 on the assumption it might be somewhere around that speed: there's not really much use in those, but there are more exposures on even a roll of 120 than you need for this purpose, and better to put them somewhere they might help you make a decision.
Make sure that the exposure time does not go below 1/2 second, or a second at the outside so you don't have to think about reciprocity failure. This may limit you at the low-ISO end.
Make sure you know which frame was shot at which ISO!
Now process the film normally, and look at the negs (it will help if you have lots of good negs to compare them to, and the serious thing would be to expose some new film at the same time with the same scene & camera for comparison). There will inevitably be some fogging, but with luck somewhere in that range there will be something which is a reasonably decent neg: use the ISO for that frame for the rest of the rolls. If there's not a reasonable neg in the sequence the chances are the film is unusable.
You mention that people might like to use this stuff for display: I'd encourage them not to! I have various interesting film boxes (like the last roll of Kodachrome I ever shot), but keeping film for display is just a waste: it should be used while it is usable. Obviously this is just an opinion however.