About a month ago, I replaced the oil with some Penrite HPR 30 20w-60 (recommended by mechanic due to a bit of oil consumption) and since then, the car just hasn't felt the same.
Feels kind of rough and very sluggish off the line. It's not until about 4000ish when the car feels like it actually has performance. You kind of have to push it a bit harder up hills as well.
Fuel consumption has gone up to 17 l/100km. That's with sedate driving (was 16 l/100km with similar grade oil and mixed driving) and if it makes any difference, it rolls a bit faster down hills in neutral, compared to before.

So, does all this sound like the result of high viscosity oil or could it somthing completely different?

Cheers

Way too thick for the cooler months. Stay with a 15W as you need to get it round quickly when cold. 20W in the cooler weather will take too long to get right around and too long to warm up. Drop it to a 15W40 or 50 as an absolute maximum. I use a 10W40 LPG oil on a worn EB xr8 engine with vapour LPG injection and pressure went up two notches with the second change(I change oil every 4000km nowadays...)

Don't put a flush through it as it will dislodge the grime in one hit. Rather do a change with a filter and run it for 1-2000km then drop it and replace. It worked for me...

Hooroo

Yeah, as mentioned before, 20W60 is a bit thick for your climate atm down there.....would be OK when summer heat waves hit though.
Original spec was 15W40.

Cheers guys, chucked some hpr 15 (15w-40) in today and she seems to go a lot better!

I've got a 96 EF and my mate/mechanic suggested upping the viscosity (15W-40 is the recommended). Ended up using the same oil and it sounds healthier than before so eh....

Then again on the weather issue I live in Port Hedland and its bloody warm all year round

Valvoline Dura Blend 15w50 is a great oil for the price and gives great oil pressure!!!