I went without shampoo for a few weeks too, and at first it did get that clumpy, oily look, but then over time as the oils distributed that smoothed out and it ultimately helped control my friz.

But after a month or two it seemed to reverse course and started looking oily again, started itching uncontrollably, and worst of all my scalp started to be covered in a sebaceous grime that water alone couldn't wash away.

So now I shampoo maybe once or twice a month, and I use a generous amount of conditioner every time I shampoo. Any more frequently than once a week and my scalp gets flakey and itchy, but a couple times a month I find is the right balance for my hair (which again, differs for everybody).

Then while my hair is still damp after showering (and detangling with a heavyduty comb), I pat some argan oil into the roots and use a bristle brush to distribute it towards the ends. That helps control the frizz, otherwise it would take a few days to smooth down after shampooing.

The stuff I use has a keratin/biotin complex but I'm not sure how much that actually does. I've also used stuff with collagen but I haven't really noticed a difference. This stuff has a consistency like honey though, so it's not just straight argan oil. I find the added viscosity helps me use a tiny bit and get it distributed nicely. I've tried using straight oil before but it's hard to get full coverage without using too much.

Any other oil would work, really: olive, avocado, almond, jojoba, linseed, grapeseed, etc.. Just use a light amount. You could even infuse it with nettle, oats, or other herbs that are good for the hair. Lanolin too if you want to get fancy. That's all extra though, the main thing is just a light coat of oil after shampooing can really help nourish and rehydrate the hair and scalp.