Lubrication - where, when, what, how much?

I am very confused and I hope someone out there can help!

I do between 12 and 20 miles of commute/town riding in the week, and
sometimes longer rides at weekends (50 miles). The usual routes lie
on the roads, but at present a key section is closed, and the
alternative involves a muddy path across a field. The bike is a solid
frame MTB running on semislick tyres and without mudguards.

I have just worn out the entire transmission (?) - gears front and
back, chain and little diddy wheels in derailleur. And I have been
told what must be the third or fourth version of "How one should
oil/clean a bike". This latest one being that I should brush down
with soapy water after each ride, rinse and relube practically the
entire bike with light oil. Whilst I see the virtue in this, I fear I
am not naturally inclined to be that virtuous.

What should I lube, with what, when and how much? Does this need to
change when the route returns to being entirely on road? I do not
have the bike mechanic's zeal for a perfectly clean machine, I just
want the bike to work well and not wear itself out too fast.

Eagerly awaiting your advice,

Sarennah
[in and out of lab, and posting via Google, please be patient]