There are major differances between DOT 3 and DOT 5. DOT 3 is a petrolum distilate based fluid and DOT 5 is silicone based. If the rubber parts of your system are made for DOT 5 and you use #3 you will ruin the rubber parts. If the system was meant for DOT 3 and you use #5, you again will ruin the parts. DOT #3 was developed primarily for late model automotive applications, were heat build up was not as apparent. Remove the master cylender cover on a high milage auto the fluid will most likly be black in color. This color change is from the rubber componets breaking down in the system. DOT 4 was a derivitive of the silica and the petrolum based fluids this withstood the heat but caused premeture wear of the rubber parts, and was also more hydosclastic, witch means it would attract mosture at a higher rate contaminating the system. Allthough it is still used today. Dot 5 is a true sinthetic low hydosclasticity, and non corosive to the internal metals. BE absolutly certain of what fluid the manufacture recomends and use only that fluid.