Generally engine oils and hydraulic oils are formulated using different additives. For example, engine oils have high detergents/dispersants (shown by high Ca or high Mg) and antiwear packages (high Zn and P). Hydraulic oils on the other hand do not have detergents - mainly antioxidant and antiwear (can be zinc based or zinc free. Perhaps there is enough hydraulic oil getting into the engine oil to dilute some of the detergent package. By Knowing typcially how much of each additive the engine oil contains, the dilution may be worked out by the reduction in add pack eg Ca goes down. Also knowing the KV of the engine oil and the KV of the hydraulic oil, you can work out the dilution factor. Compare the KV dilution to the add pack dilution for some confirmation.