low-mileage 1998 LeSabre: p0171 code on/off ... and oil leak

Here are the pertinent facts about this Buick LeSabre 1998 Limited -- which I got in August with 38,000 miles and now has about 45,000. I am new to DIY on vehicles and bought this in part because I want to learn and willing to try with so little invested (paid $2,400 for it).

1) For about the last 2,000 miles, ever since the only extended trip I have taken (250 miles each way over Christmas), the p0171 code showing lean bank 2 comes on and off -- 65% of the time on, 35% off. It always comes on within 5-10 miles of filling up the tank. It almost always disappears at about a 60%empty/40%full.

2) Last oil change, it was clear that oil is being lost somewhere but NOT from the gasket or drain pan. Seems to be in internal leak. Absolutely no oil drips onto driveway, parking lot, etc. So it must be leaking very lightly internally while engine is running and in motion.

3) Recently, the CHANGE OIL SOON light has come on and stayed on -- according to the manual, the computer does detect when oil has degraded. Although it was not the CHECK OIL LEVEL light, I did see was almost a quart low and so I have topped it off.

4) While I believe this could be the intake manifold gasket issue, I have sprayed and sprayed with carb cleaner and noticed no difference in rpms. In general, the car does not idle roughly -- seems very normal to me. No real issues at all, in terms of noticeable symptoms.

Appreciate any further thoughts on this. My other issue is that relationship with very trustworthy regular mechanic for years has been very much -- "you just do whatever it takes". So while certainly it's possible for me to adjust the relationship to "diagnose for me and i'll fix it myself," his is not the type of shop where that's customary.