Hi
Oil isn't a permanent barrier. We say that it "seals" the moisture in, and to an extent, it does (when compared to not sealing at all), but it is still porous. It isn't some impenetrable shield and it does "wear off." Otherwise, why would we need to do it often? Also think about when you tie your hair up at night; There's a reason you have to wash your pillowcases and scarves/bonnets. The oil doesn't all stay on your hair. Some of it does stay on your hair, which is why if you go a long time without washing, while adding oil/products frequently, you will get build up.
Oil builds up faster in my hair than silicone products, so although I love my Hot 6 Oil, I don't use it everyday. What's really working well for me lately is spraying with Profectiv Damage-Free leave-in and while it's slightly damp, applying Profectiv Long 'N' Healthy Mega Moisturizer. Makes my hair really soft. I usually shampoo exactly once a week unless I've recently used Smooth and Shine Gel which builds up a lot, in which case I'll shampoo the day after I use it. I'll sometimes do a conditioner only wash 1 or 2 times during the week, depending on how my hair is.
I just bought a clarifying shampoo yesterday. I hadn't clarified since I was home this past summer. My hair didn't feel weighed down or anything, I just wanted to see how my newest deep conditioner works right after clarifying (I just tried it this morning--nice!)
I don't think you should necessarily have to AVOID silicones in products unless you think it's really causing a problem. Being silicone-base, and just CONTAINING silicones are two different things. Serums are silicone-based--some, in fact, are completely made up of silicones. Many moisturizers contain silicones, but they aren't the main ingredient. If you think silicones build up on your hair, choose products that, if they contain it, have the cones towards the end of the ingredient list.
HTH!