Another vote for agave syrup (sometimes sold as agave nectar), it's great stuff. I haven't tried it on porridge, I like mine plain these days.
I used to put a bit of molasses on my porridge. It adds sweetness, but it also adds a strong enough flavour that I don't think you'd want as much of it - providing you actually like the flavour, of course, which I personally think is fab. It's a bit healthier than caster sugar, it's got a good mineral content as it's not heavily refined in the same way.
I know squat about sugar substitutes, stevia and the like, but presumably someone else here does?
If your level of sugar is bothering you, could you simply reduce it gradually and see how you get on? And as a few people have said, the odd teaspoon of sugar is unlikely to do much harm, you need to feel happy and satisfied with what you're eating. Plus it's entirely possible that if you are reducing sugar in your diet overall and generally eating more healthily, you'll find that you gradually want less sugar on your oatmeal anyway.