It’s a very common question – but it’s hard to get a straight answer.
I’ll try to give you one.
On your particular car.
And how you drive it.
First, let’s consider the car you’ve got.
The older it is, the more often you’ll need to change the oil. Especially if it’s older than circa late 1980s and has a carburetor (rather than fuel injection) and does not have a computer.
Carbs have many virtues, including adjustability and fixability (fuel injection components are generally not adjustable or fixable; they’re pre-set to work a certain way and you replace not-working parts with a new parts). But, they are sloppy with the gas. Carburetors are basically little metal (alloy, actually) boxes filled with liquid gas. The gas is held in a reservoir called a fuel bowl – and it’s from here the fuel (along with air) gets sucked by vacuum into the engine.
These fuel bowls are prone to seep and leak; liquid gas washes down the engine’s cylinder walls and other metal surfaces (taking with it the protective film of oil that would otherwise lubricate the moving parts) and ends up mixed with the oil in the crankcase.
Oil and gas – just like oil and water – is not a match made in heaven.
Reagan-era (and older) cars with carbs also do not have computers to monitor – and automatically adjust – the air-fuel ratio, to prevent the engine from running over-rich (too much gas in the mix) or lean (not enough gas in the mix). Less than optimum combustion means more combustion byproducts, which means the oil gets moregunk in it sooner. This, in turn, comprised its ability to lubricate (and keep clean) the engine’s innards. Which resulted in more rapid wear and tear… and so on.
An aggravating factor back then was the oil itself. By current standards, the stuff we put into our crankcases even 20 years ago was the equivalent of Folger’s Crystals vs. Starbucks coffee.
That’s why – in the past – the rule of thumb was to change the oil every 3,000 miles or so.
But cars built since the mid-1990s through today have fuel-injected engines. The gas is fed to (rather than sucked into) the engine in a carefully atomized spritz. There’s no reservoir of liquid gas sloshing around in a fuel-injected system to dribble into the engine; also, the computer monitors and adjusts the air-fuel ratio for optimum combustion. If something’s not right, you get a “check engine” warning to let you know. Ignition systems are also much-improved (and engine build tolerances much tighter now than in the olde days). Contamination of the oil is thus much less – and takes much longer.
Oil quality is also much improved. Synthetic and semi-synthetics are now commonly prescribed. These offer more protection, especially under extreme conditions – and they maintain that higher level of protection for longer.
Result: Longer intervals between oil changes.