Since the tank shape on the IV is quite a bit different than on my ES I can only relate how calibrated the dipstick on mine.  The first time I fueled the plane I  started from dry and checked the depth every gallon or two from there till full.  It takes 14 gallons before the fuel reaches the bottom of the tank directly below the filler.  The fuel quantity is, of course, non-linear with depth from there until full, so I graphed the curve.  On every preflight I check the quantity against that curve.  Simple process if you start from zero fuel.  If you know the total, but haven't done a partial-tank calibration (that's what I did with a previous plane) you can measure the depth of whatever fuel there is and then start filling a little at a time.  Subtract from the full quantity to get the partial fuel calibration.  The dipstick I have always used is the transparent "soda straw" sold by Sportys and all the others.  Works well.  Of course, I can't measure fuel quantities less than 14, but I'm not likely to go flying with less than that in a tank.  Certainly the IVP tanks are longer and thinner, so there would probably be more uncertainty in the measurement - but that's not a good reason not to do it.

regarding the second question, I don't think there is an absolute answer to how often to change the oil.  As I recall most OEMs have recommended 25 hours without a full flow filter and 50 hours with.  I had one airplane where they said 50 hours to change the filter and 100 hours to change the oil.  I changed both at 50 just because it was easier.  It also depends on usage and how much oil is lost between changes.  One could argue that if the engine burns enough oil it gets changed all the time, which is a legitimate argument.  except that high oil consumption usually brings with it high blowby, increasing the contamination of the oil.  Some diesel trucks have used a system that pumps a small amount of oil into the fuel so that the oil never has to be "changed."  I believe all large marine engines use that system.  I think that 50 hours is a conservative number for an engine in good shape.  I personally start looking for an opportunity at 50 hours and then don't get worried if I go over 10 or 20 hours.  If I had 40 hours on the oil and was about to leave on a 20-hour trip I would certainly just go and change the oil when I got back.  Probably one of those "hockey-stick" curves - doing it early probably doesn't help, but waiting longer and longer will do progressively more and more harm.  Running LOP will increase oil life as there is less lead contamination.  The easiest way to tell?  Look at the oil - if it is ugly, time for a change (recommendation from an oil expert I knew).

Gary