Can't help you on the oleander. However, I bought a 400 watt metal halide light specifically to overwinter tropical hibiscus. They didn't turn yellow (except when they had assorted pests, which was much of the time), but they did get leggy. It is possible that the HPS makes them leggier than the MH; that is true for some varieties of plants, but I've not been able to find research specific to hibiscus. I have overwintered a hibiscus next to a south-facing sliding door with much better results than the MH light. Now, you can put ONE decent sized hibiscus directly under the light, about a foot away, and it will do quite well. But that's a LOT of money to spend on a light for just one plant!!

To stay nice all winter, hibs need an enormous amount of light. You can keep them alive over the winter under lights if you give them a lot of attention, but you won't get many flowers and they won't look great.

I finally gave up on the overwintering. I spent too much time treating bugs, pruning, dragging the plants in and out of doors, repotting, etc., and I still wasn't happy with the results. I decided it was easier and cheaper to just buy new each spring and treat them as annuals. I hate to do it, but it just makes the most sense.

Now if you have a lot of time, patience, and money, or it's just plain fun to you, go for it! Once I started working with plants professionally, I no longer had the time or patience to baby the hibiscus at home all winter.

are you watering your hibiscus too much in the winter? I have a couple of huge lantern hibiscus I bring in and they do ok under the light, not directly under it, kind of off to the side, as long as they are not watered too much. I hit them with 20-20-20 at 150 ppm every other watering, leach with fresh water in between. They didn't grow much, but they didn't lose leaves, etc. They did bloom a little bit on and off. I let them dry out almost to the point of wilting in between waterings. Maybe that will help.