A lanolin based product would be really good.
However, the best stuff to use would be proper Rust proofing, or cavity wax as it's commonly known these days. Fishoil is a thing of the past. It is effective, but technology has well and truly made fishoil redundant.
If you know someone that buys Wurth products (not available retail through any shops) Get them to get you some of thier cavity wax. There are two types, a aerosol type that is a bit thinner and would be the one I would recommend and it also comes with a thin tube perfect for squirting down tubes etc and being thinner it's better for not adding too much extra weight.
They also sell 1L tins for use in thier sprayguns. This is a bit thicker and intended for large rust proofing jobs. But unless you have one of the specal guns you can't apply it.
The Wurth cavity waxs are by far the best products I've ever used!