[legal/regs] Does the hibeam and the lobeam have to part of the same light source?

My 1st gen has dual filament headlights. Single bulb does both hi beam and lo beam. Can I run two separate bulbs to serve the separate functions? Anything by DOT saying I cannot?

 

Not in the original housing, no. The filaments are both placed precisely to focus correctly in the reflector. Two bulbs, even with their glass touching, cannot properly fit and focus. If you install am aftermarket set of headlamps that has separate low and high beam reflectors/projectors that's fine (but probably not a great choice given the offerings from spyder/depo etc).

Crashnburn80 has an excellent thread stickied at the top of the 2nd gen subforum titled the "Ultimate Headlight Upgrade." He has tested a large number of H4/9003 bulbs, and you can see their performance comparisons vs the stock osram long life bulb. While the results will not be 1:1 in your firat gens lamps, Crash has also posted a couple tests in his most recent 1st gen truck build thread, showing that even the OEM lamps are not too terribly far behind their larger, more efficient 2nd gen brothers, with a good bulb and relay harness.