First
Your Glim-Lamp burns fuel at a constant rate over time. The rate of total fuel drain over time appears to be (fuel drain over time from speed) + (fuel drain over time from Glim-Lamp).
Why use your Glim-Lamp?
It's your biggest tool for preventing the rapid accrual of terror!
A Bit About Terror
Terror is the numeric statistic that can make you lose the game. It goes up by one whenever the little circle on your Terror bar fills all the way up with 10 "ticks."
The rate of terror tick increase appears to be determined by the net value of conditions that are effecting your ship. It's a little complicated, because there appear to be both positive and negative conditions, and some (like if you're in total darkness or lit by a light source) are mutually exclusive. It appears to be a mixture of light level and proximity to shore.
I'm not sure if this is how the game actually tallies it in the background, but using these principles you can accurately predict at what speed a set of conditions will increase terror.
Positive conditions and negative conditions appear to cancel each other out.
If your total is 0, you gain terror ticks at the "yellow" rate, slowly but surely.
If your total is -1 or less, you gain terror ticks at the "red" rate, rapidly; twice as fast as "yellow."
If your total is 1 or more, your terror level is "green," and the ticks actually recede, back down to zero, but your overall Terror stat will not decrease.
Positive Conditions
Glim-Lamp on
Ship lit by light source (stack-able!)
Ambient light radius (not Glim-Lamp beam) touches shore line. (This may also apply to external light source pools, but I haven't felt like the testing was consistent due to the way the game updates terror accumulation rate at longish - 1 or 3 second? - intervals.)
Negative conditions
Ship not lit by any source of light.
Ship is in fog or storm.
Shoreline not within ambient light radius.
Example 1:
You are sailing through a fog bank in the middle of the ocean with your glim light on.
-1: No land in your ambieant light radius.
-1: Fog Bank
+1: Glim Light
-1: Total - Your terror is increasing rapidly ("red").
Example 2:
The same as the above, except you are hugging a shoreline. This additional +1 means your terror is still increasing, but at the normal ("yellow") speed.
Example 3:
You are traveling through a buoy's light pool near shore at the London Docks†:
+1: Shore within ambient light radius
+1: within buoy's light radius
2: Total - There are no negative conditions here, and terror ticks are reducing ("green").
†Note that some of the buoys here appear to be cosmetic and don't count as light sources.
Example 4:
You're sailing without your Glim-Light through a fog bank, near a buoy, and you're in a lighthouse's beam.
-1: no land in ambient light radius.
-1: in a fog bank.
+1: from buoy.
+1: from lighthouse beam.
0: Total. Terror is increasing at the normal "yellow" rate. If you turn on your glim lamp, however, and you'll be back in the green!
Strategy
You can get most of the way across the map while accruing very little terror and without burning too much extra fuel.
Plan your routes to hug the shorelines as much as possible.
Remember the easy or free sources of terror reduction. Plan routes to include one or two of these if it's a long route.
Hit the light buoys and light house beams (when lucky).
When skimming the shoreline, allow the yellow terror ticks to build up to 7-8, then turn on the glim light until they are back down to 0-2. Alternate as necessary. There is no use burning your light for no reason!
Route around fog banks and storms (which also slow you down, increasing your Glim-Lamp burn time).
Very fast ships can better justify leaving the shoreline, because their Glim-Light burns will be inherently shorter and use less fuel.