I got about 2 gals of new oil+ hyd. fluid did a 50-50 mix filtered it and put it in the almost empty rear tank. When I switched to the rear tank the fuel filter light came on. Switch back to the front it goes off. I keep adding diesel to the rear, switch to the rear and light comes on switch back to the front the light goes off. The oil was new not waste. Truck runs fine both tanks. 94 idi turbo. Any clues why