Rather than throw the old fuel out consider adding more ether.
Make up a small mix of 80% ether and 20% castor.
Add a portion of this to a known volume of old fuel.
You will not be diluting the oil content below 20%.
Keep adding the fresh portion until the old/new mix starts to run ok.
Even if you have to add half and half, you will still end up with a mix of about 40% ether (assuming the old mix had none left).