As Nigel says, lots of good info and possible causes listed here already.
If it were I, I'd think twice about chucking them as oils can last for many years. I have tubes of WN oils given to me by a fellow artist that are so old they still have the old WN logo on them. And yes, they are as usable today as they were when first purchased. But, as I have to do, you may need to squeeze out a bunch to get to fresher material.
As the other guys have suggested, it sounds like you may have the wrong thinner/paint combo. Have you tried good old fashioned turpentine? That is the classic thinning medium for oil paints. It's available in an odorless variety, at least here in the States.
Best of luck with this......
Cheers from NYC,
Michael