I wouldn't give the oil. Cats are meat eaters and don't digest corn and corn bi products. You will do more harm than good.
What kind of kidney problems in the past? How were they kidneys diagnosed as having problems? Blood work? If so, just so you know there has to be close to 70% loss of kidney function for it to show up in the blood work. So by the time it is showing the renal values in the blood work as elevated the kidney disease is already pretty far along. An abdominal ultrasound with an internal medicine specialist is the way to go if you can.
What you have to watch out for in renal cats is that they get enough fluids. Often times the vet can show you how to give sub q fluids to your cat so you can do it at home. This really helps renal cats live full and happy lives.
Do you know for sure your cat has kidney issues?
Other causes of weight loss or just general skinny cats can be many. One of the more common is IBD, or intestinal lymphoma. Both can be treated but require an endoscopic biopsy for diagnosis.
Try going to the website called IBDkitties.com and look around there. There is alot of good advice and information about diet etc.
And make sure your cats have access to several areas of clean water to drink.
Hope your cat is doing okay.