TR-6 oil pressure - 3 questions

[gdk@thumper.bellcore.com (Gabor Kiss)]

I recently changed the oil filter and have noticed a sudden drop in
oil pressure when hot. It's now around 30 PSI @2000 and 45 PSI @3000

1) Is it possible for the filter itself to affect oil pressure? Is the
pressure tap upstream or downstream from the filter?

2) How much difference could I expect going from 10-40 to 20-50 oil
(is it worth draining out the oil that's in now)?

3) (OK that's 3 questions) is this a worrisomely low oil pressure or
not? 

                                        Gabor