The Skydrol 500 series of fluids has the longest service history among phosphate ester products. The first version, Skydrol 500, was introduced in 1952. Steady improvements to the formulation led in 1977 to the current version, Skydrol™ 500B-4 which contains the same breakthrough anti erosion additive and acid scavenger found in Skydrol™ LD-4 . Skydrol 500B-4 is the most worker friendly of the aviation phosphate esters. This has given the product enormous popularity for use in work shops and indoor test stands..