what is the difference between SEL and OAT entries in the FMC? i.e. what would i enter into SEL and what would i enter into OAT...

I may be wrong, i haven't anyh reference material around me at the moment, but OAT is the actual Outside Air Temperature, and SEL is the SELective/assumed temperature you can enter to reduce engine wear as they'll use a lower trust setting

It's so you can carryo out a de-rated take-off, do a search for that and you'll find loads of posts explaining how.You'll need the charts that came with your download and the length of the runway. It makes t/o a lot easier too.Glen