As far as I understand, you want to do this when switching from dino oil to Mobil 1, or whenever you feel like it, but especially when switching.
As far as I know, you go buy that STP Engine Treatment for like $6, when you are ready to change your oil you dump it in the crankcase, run your car at like 1500RPM for 15 minutes or so, drain your oil, next put in a full amount of dino oil, (now do you run your engine again or just drain it straight out again?), then after draining it put your Mobil 1 in, right?
I think the problem of retaining oil has to do with the fact that an sr20 oil pan is so shallow, the least bit of sludge build up will affect oil levels.
The BEST way to remedy this is to periodicly remove and clean out the bottom of the oil pan.
Why would oil build up on your oil pan if you change your oil regularly? The only reason I would see this happening is if you changed your oil when your engine is cold. I try to do mine when it is hot. I thought that some of the qualities of synthetic oil was that it would "clean" some of the old oil out. Should I be concerned with sludge if I have used Redline from my first oil change and change my oil filter every time, or would it be ok, when I change my oil it seems like it does not loose any of its viscosity.
The reason sludge will build up over time is because there will always be a small amount of old oil and sludge (the dirt, fuel, moisture,etc. that's in the oil) at the bottom of the opil pan. If you remove your oil pan to clean it out, you'll see what I'm talking about. The oil pump pick up is only a few MM above the bottome of the oil pan.
Now for me, I like to change the oil while the engine's cold. Two reasons: 1) I don't like second degree bunrns. 2) I figure with a cold engine, you'll be able to drain ALL the oil out because it's settled at the bottom of the pan.
Don't worry about what brand of oil you use as long as you use the recommende oil viscosity (5W30), and change it at the recommened intervals.