I've found denatured alcohol is an excellent solution for removing oily and sticky substances. I've used denatured alcohol for decades; it's one of my favorite go-to solutions for sticky situations. (Yes, that was meant to be punny.) I keep some handy in a small bottle (transferred from large can I keep in the garage). Mineral spirits has an oily residue when compared to denatured alcohol which dries quickly and with no discernible residue.
Why not do a test spot? Or simply clean all relevant surfaces with denatured alcohol--you can use it liberally exactly because it dries quickly without leaving any residue--and apply the silicone sealer. Make sure to use clean, lint-free rags. Anything else and you could risk leaving fabric or paper lint behind thus compromising the seal. Water is persistent and insistent--anywhere it can find a way in, it will and it will keep hunting until it does. Use clean sections of the cloth to ensure you're WIPING OFF the oil and not simply smearing it around. Because you've got a big gap, I've read instructions to first 'block' large gaps and then apply the caulk. Not sure what the recommended material is for filling in/blocking the gap (it's generally called a gasket, yes?) so you'd have to research that.
Another option is to use rubber-based product. It's great for keeping water out. I used RedGard (purchased from Home Depot) to retroactively waterproof the floor of a tiled shower where I didn't have the option of installing a membrane (hack contractor job). Four years later and it's still working great. RedGard was relatively expensive, but I only had a small area so it was well worth the cost to ensure a waterproof seal. I'm about to use rubberized caulk (Flex Shot) to close gaps in the door seals to our daughter's car that gets water inside when it rains. I hope it does the job! I'm also using the Henry line of elastomeric products to seal and waterproof our 90 yo metal roof and used their 100% silicone reflective coating (Tropi-Cool). So far, so good. Henry products are available at Home Depot and you can check out their caulks at Henry.com. No, I don't work for Henry nor am I affiliated with any company in anyway. I'm merely sharing my knowledge and experience; that's how I have learned what I know today: Reading information people have taken the time to share, researching the data & options (to death sometimes), and then checking it out for myself.
Here's something I found when asking RedGard chat support if they had a caulk version. They didn't, but sent this link: http://www.custombuildingproducts.com/TDS/TDS-100.pdf
Hope this helps.