2001 Mitsubishi Eclipse while driving my glass instantly st

My 2001 RS suddenly ejected water vapor which covered my glass (on the exterior of the car) while at speed (70mph). The car just had an oil change 3 days prior (castrol synhetic, that's all I use on all my vehicles) and last year I installed a "Forced Air intake" from K&N. The assumption is the thermostat but thee appears to be minor oil seepage around the valve cover. There was no noticeable leaking while the engine was running or after it shut off, hoses appear intact (not collapsed). While driving home had to keep the heater on or it would spike. No loss of power, seems obvious but I'm many years away from being a under the hood savvy.