VICKERS FR5-10-F-0-4.00

Fixed pressure compensated flow regulators are built for exactly that situation. When load-induced pressure swings would otherwise throw off your flow rate, the compensator corrects for it automatically and keeps output steady at the set rate. On this valve that rated flow is 23 litres per minute, so as long as that falls in the range your circuit needs, you'd get consistent delivery even when the load is varying. The valve is also rated to 350 bar and can handle system flows up to 350 L/min, so there's plenty of headroom on both pressure and volume for a demanding harvester application.