Yes, the pressure at the bottom of a 3-feet-deep holding tank would be different if the tank held motor oil instead of water, even if the volume of the liquid and the height of the tank remained the same. This is because the pressure at the bottom of a liquid is directly proportional to the density of the liquid.
Motor oil is less dense than water, which means that for the same volume, it has less mass than water. Therefore, it would exert less pressure on the bottom of the tank than water would. Specifically, the pressure at the bottom of the tank would be equal to the weight of the liquid above it, divided by the area of the bottom of the tank. Since motor oil weighs less than water, it would exert less pressure on the bottom of the tank than water would.
It's worth noting that the difference in pressure between the two liquids may not be very significant, especially for a relatively shallow tank like the one described in the question. However, the difference in pressure would still exist, and it would depend on the specific densities of the two liquids involved.