There are as many opinions on oil as there are drivers. I bet you could spend all week reading the oil related threads in the archives. I don't have a manual around for an 88 but Bentley is nearly always right, if you don't have an owners manual (they've got the useful little chart). Offhand I bet 10w30 or 0w30 is correct or close most of the year.
I use 10W40 Synthetic in my Turbo in Seattle, pretty much year round. Our weather is pretty temperate, no real need to change weights unless I am planning on spending some time in the mountains.
I've heard the argument that older engines require higher weight oils, but I'm not convinced. Personally I think the wear that occurs when you start an engine on 20W50 in the dead of winter is pretty bad. My dad uses 20w50 in everything, yet he's got an ill-maintained 89 GM 350 with 230k. Who knows what is right.
Whatever weight you choose, keep changing it often and you should be good.
Living in Seattle I vote bumpers for spare parts.

Logan
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1981 242 GLTi 1967.5 122S
Seattle, WA