I personably do LI, then oil, then gel.
I seal with different things - jojoba oil, SM elixir, but my HG is mineral oil. My hair is pretty porous so I lose a lot of moisture. When I use MO, it helps hold moisture in, keeps tangles at bay (my nape can be RIDICULOUS), and hold excess humidity out (when that applies!)
I have no issues with build-up, even when co-washing. I do clarify occasionally, and I use low-poo once a week, but I never notice any issues.
I just do a few drops onto very wet hair. My hair is long and dense, but I use 3-5 drops on my nape, and then 3-5 more on my ends. (of course some moves up the hair shaft when I apply gel, I'm sure.)
Sometimes it helps to mix the oil with a little LI, just so it's thicker and easier to see where I'm puting it.
I use the same "system" no matter what type of oil I'm using.
2c/3a/3b, Fine/med, Dense, high porous, & normal elas.
Lo-poo - Renpure Keratin, SMR, Giovanni SaS
clarifying - mix Johnson's H2T with low poo
Co-wash - Suave Nats. clar. or coco.
RO - GVPCB, Ion Eff. Care, TN.
LI - SMR, Cure Care, or RO
Seal - MO or Jojoba
DT - + honey, ACV, & EVOO to SM masque & GVPCB
PT - Gelatin!, CNPF
Styling - FSG, AIF, BRHG, LALS/PS, GF curl spray gel, SM Souffle/Smoothie, LOOB
Oils - MO!, EVOO, jojoba, GSO, SM Elixer, EVCO, VO5 Hairdressing