When I was running my 94 +8 the best advise was to use a standard 20 - 50W. Halfords still sell Castrol GTX which fits the bill. Apparently it's not such a good idea to use synthetic oils on the old (design) Buick block. Go-Mog has a fair bit of useful information on this topic. Enjoy your V8.
Please see attached link :- http://www.gomog.com/allmorgan/plus8oil.htm

2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev