Wrt your first sentence, a 40 grade oil will be better than 30 at silencing a lot of troubles, it will also increase pressure through all parts of the engine once it gets to temperature, including the oil pump (and filter). You may consider changing the latter more often.
My fix (on the Mk7) was to force the old brake fluid back up the pipes by squeezing the calipers hard with a large set of grips, then suck out the incoming fluid from the main reservoir before refreshing with new.
As above all good after five years.
I can't recall the max rpm, but I could get up to slightly over 40MPH in top/fifth gear on a good day.
Farhad suggestion of the TPS could be good though I always got a fault code before replacing mine a couple of years back. I did post images somwhere of the track wear and lived with the fault...
It sounds like a limp mode because it is. I had times when a code didn't flag up and switching off and restarting (three times ?) clears whateve may or may not have been on the dasboard.
My guess would be an electrical issue, damp in the fusebox - or any one of a number of connectors seeing it...