Au2 2003 tickford xr8 200kw

[AGE1%]

I just bought new topgun standard leads and platnium sparkies(plugs) from autobarn, and installed and they seem ok. Before that on the same day i went to a recommended shop and asked for good oil and filter. The guy gave me 20/60 oil and a cheap z9 filter (as i found out after i fckd er a bit! coopers make good filters aye !). The recommended shop didn't have the leads and plugs thank god. Anyway the car seems pretty good except it seems to labour and flatten out in certain rev ranges. I have used the wrong oil have't i? it seems too thick? I am humbley asking what is the best oil to use in my pride and joy? I gave her 5.5 ltrs as the manual said but it seems too much on the dipstick. Any feedback would be muchly appreciated... cheers... Age...