If I wanted to add an oil cooler to my 1981 CB750K

[Cabreco]

It seems simple enough. 2 lines, a cooler, and an adaptor between the block & filter cover.
I see many complete cooler units on EBay cheap.
In order to do this, what year & CC motor donor bike should I get the parts from?

[Cabreco]

Also heard something about sandwich adaptors?
Anybody?

[sgtslag]

I have a PDF article on this very topic.  I installed an oil radiator off of a 1980 CB900, on my 1979 CB750K, complete with an oil thermostat.  The process is detailed within the PDF article.  Let me know your e-mail address, and I will send it to you, if your account's filters will allow a 736 kB PDF attachment.

The oil cooler system dropped my oil temperature, riding two up, full saddlebags and trunk, at 55 MPH, in 77-87 F air temperatures, from 250 F, down to ~210 F!  This will extend the life of the engine dramatically (hot oil tends to dry out, and harden, gaskets, and seals...).  Otherwise, just run synthetic oil in your engine, and be done with it.  

With regards to the finned sandwich adapters, I do not know how much cooling they will accomplish.  The only way to find out, is to install an oil temperature gauge, and test your bike with/without the finned sandwich adapter.

I tested my bike, with/without the thermostatically controlled radiator, and I achieved a legitimate 40 F drop in oil temperature, which is consistent, and measurable.  YYMV.  Cheers!