5.0 grains of Unique has been a standard pressure load in .38 spl. since 1935 (Phil Sharpe's manual) Notwithstanding the plethora of lighter load data (much of it linked to swaged bullets which are sometimes loaded light to prevent barrel leading issues) good manuals have continued to list 5 grains or a bit more as standard pressure loads all the way thru to the present. 5.4-5.5 has likewise been listed in MANY sources as a +P loading with 158-160 grain bullets. A few sources from the 38/44 era 'til the present have presented 6 grains of Unique as a max load with 158-160 SWC.....most of those sources don't provide pressure data but one that did had the 6 grain load as 25kCUP-which is over SAAMI but under Proof pressures.
My standard .38spl load is 5 grains of unique with a 160. It leaves very little to be desired.