I had a 1986 190 16V, that recently sold in Europe. I used RedLine oil products (pure synthetic), I also live in the Seattle area and would use 05/30 and 10/30. You will see an oil gage tempeature drop immediately after your first oil change. It has been an excellent product and would recommend the product.
You may experience some oil leakage around your valve cover seal if you have been using regular oil products. You may have to change your gasket, but this would be the case if you went to any pure synthetic oil product.
Hope this helps