Oil Furnace/Oil Tank questions

This is my first time with an oil heater. I have electric space heaters at home, and electricity is cheaper here than the eastern shore, more like 10 instead of 14+ cents per kwh. In 2000-2002 I rented upper floor of an old house but that had propane (and automatic fill ups) and cost was not bad. Now I have to do the refills but concerns about costs, efficiency, and how well maintained have entered my mind. Ok:

#1 - Too late to do anything about, but the minimum is 100 gallons (which costs $375) and I was told my tank is about 275 gallons or something like that so 100 gallons should be *more* than 1/4 yet the level (which had been at "half") only went up to 3/4.

It's also unfortunate that, while I can read my electric meter at any time to see where exactly I stand 2 weeks or 1 week after the last billing cycle, here I can't even tell to the nearest 25 gallons, and maybe barely to the nearest 50 gallons by visually looking at the level.

#2 - Before the latest concern (items #3 and #4) I wanted to ask if you have any idea what this sign means, in terms of how efficient/ok the furnace is. It seems to say "stack temp 4000" [yes, presumably "400 was meant!], "draft" is either "0" or "2" depending how you read it.

Next, the inspection was 2006 and 2010, I guess that's ok, 2 years ago (2010) and "efficiency" seems to say "811/2%" what on earth does "811 / 2%" mean? Finally I do have a CO detector at home, but this sign refers to CO2 not CO, hard to read in the photo but I looked up close. You probably know this from experience anyway. Photo:



So what do you make of these numbers by the inspector?

#3 I can take a photo of the furnace itself which is behind a door. It has "reset" (turned itself off) on Nov 5, 8, 11, and 14.

I will probably report to landlords (who have been well meaning and generally good but clearly this house by FAR is the least ready and with the most issues I've ever been in - and I speak usually with the 61 year old even though her mother who is 80 is the actual owner and she owns 5 houses and barely can keep up with the state of this one) but I want to collect more data and also get a better understanding.

Now before panicing about the four self shut downs, the good news is: it kept working overnight (at lower 61F) and seems to happen only when I get up and move it to 67F or 69F). One of those four days was very mild/warm (but house still needed warming to closer to 70) other day was chilly others in between. Other good new is when I reset, it starts and gets going and does not turn itself off. Nor overnight either. Just when I do it when I get up to warm things up to my waking hours preferred temps. Clues?

#4 my latest concern: Not liking the fact that I can only oder $375 at a time I tried to really conserve. My house is only 950 or so square feet AND I have kept the overnight temp near 60F **and** on top of that I don't even use two of the three bedrooms right now - they are for storage, they are at 54F when the house is at 62F for example, their doors are closed so despite heat pipes there they do not get as much heat, and so I have maybe closer to 700 or less square feet I'm heating..

And on top of that I have been using two and later three electric space heaters during many hours - all night in my bedroom where I sleep and starting in early morning and much of the day to "supplement" the oil heater, for the main house areas.

So I thought the 'needle' (the level) should have barely moved at all in the 45 days since I got the oil, it's been mild weather, not winter - lows no lower than 28F and sometimes a low of 45F or higher, usually lows in the middle to upper 30s. I got an unpleasant surprised when I looked at the furnace level yesterday. I took a photo of it today, and I have the October photo.

And tank is not small. The yellow thing is a 36" (yardstick). Front:

And side:

On Oct 4 the level was:

On Nov 18 the level was: