Linseed oil application to the inside of a steel frame
The preferred way to do this is to use something like JP Weigle Framesaver. This is an aerosol so you're more likely to get the product in more places which should be protected. And it will dry much quicker (though it has been known to make seat posts somewhat prone to slipping). 

However, companies like Surly still state boiled linseed oil and motor fogging oil are  acceptable ways to protect the inside of the frame from rusting. John Allen also discusses a product called Boeshield T9. 

You're only saving about 5 bucks probably by using boiled linseed oil, so I don't see the point in preferring it unless you already have it / have some problems with aerosols.