Back when I owned a C4, washing the engine was a time consuming chore. I made a bucket of soap and water (usually Simple Green). Got a rag or 2, a large bottle brush, a flat brush with bristles top and bottom and pumped up my air compressor with a spray nozzle attached and the pressure output set around 40 psi. Keep a hose ready also with low pressure also.
I'd use the rag or the appropriate brush to clean an area working from the top down, working in small areas at a time. Then lightly hose off the soapy water, then blow the area dry of water.
When not working around the OPTI you can use more water and air. Around the OPTI, I used less water and blew it dry more often.
Wash the motor when cold. A warm or hot OPTI will cool quickly when wet and this allows the water to be sucked into the vent hole on the bottom of the OPTI which is not good. The 95/96 OPTI's had vacuum lines hooked to the vent hole and are less likely to be damaged by water but....
After owning a '92 for 14 years and over 100K miles, I never had a problem with the OPTI except once when the humidity in Galveston County, TX stayed around 99% for 5 days and the car cold soaked in the garage the whole time. I knew the problem as soon as I tried to start it. 30 minutes of hot air from a hair dryer pointed at the OPTI dried it out and the OPTI pressed on for several more years.
