First of all, please dont flame me cuz im not the one that was doing this.
So i just found out that my fiance's father was putting only 4 quarts per oil change. i will be doing the changes from now on and will be putting 6.5 quarts from now on.
Here's my question. My future father-in-law had been doing this for about 2.5 years now and roughly 30K miles. What problems can i expect in the future for the lack of oil the engine has had?