I don't think the apostrophe is optional.... YUKOS's profits, or YUKOS' profits (singular ending in -s, so apostrophe is optional) ...
I think what etb meant to write was "so the s after the apostrophe is optional" - and it is true that some people treat singular names and words ending in s as if they were plural.Yes, use the same principles as for common nouns:
YUKOS's profits, or YUKOS' profits (singular ending in -s, so apostrophe is optional)
Temasek Holdings' profits (plural ending in -s, so no apostrophe)
Some companies already end in genitive 's, like McDonald's, so you can't change them any more: McDonald's profits.
We've been round this buoy before, Egmont, in previous threads. St James's Park is on the same central London map as St Thomas' Hospital. Newcastle United FC plays on St James' Park. Exeter City FC plays on St James Park. Not much consistency there.My authority for this is any map of central London that shows St. James's Park