Flexible oil and hydraulic lines are generally measured in 1/16" increments, so a #8 line would be 8/16 or 1/2" inside diameter. I'd consider a 1/2" ID to be a minimum to assure adequate flow through a full-flow filter. We used dual filters in parallel and 3/4" lines on the dry sump oil systems of round track race engines to minimize pressure drop across the filters.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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