We have a male cat who is almost eight years old. He will urinate in a litter box but prefers to defecate on the concrete floor underneath my oil tanks in the basement. I discovered this earlier in the year. We also have two female cats so there are multiple litter boxes. I moved one of the litter boxes next to the oil tanks and have so far tried three different types of cat litter. I have seen him urinate in the litter box and then go under the oil tanks. Any suggestions are welcome. Thanks.