However, a good rule of thumb for a 1994 Dodge van using conventional oil is every 3,000 miles or every 3 months, whichever comes first. This is a conservative estimate and erring on the side of caution is always recommended for an older vehicle.
If you're using synthetic blend oil, you might be able to extend the interval slightly to around 5,000 miles or every 6 months, but check your owner's manual for specific recommendations (if you have it). Full synthetic oils can sometimes go even further, but again, the age of the vehicle suggests more frequent changes.
Harsh driving conditions (frequent stop-and-go traffic, towing, extreme temperatures) will shorten the oil's lifespan, requiring more frequent changes.
Ignoring the oil change schedule can lead to serious engine damage, so it's better to change the oil more frequently than less frequently for a vehicle this old. Consider the 3,000-mile/3-month interval as a safe baseline, and consult a mechanic if you have any doubts.
