New car and truck owners are often anxious to protect their investment by switching out the factory fluids and converting to synthetic oil. But the common question is “when” this changeover should occur. Let’s explore when the earliest point at which the factory fills should be changed to synthetic oil in the engine, transmission, differentials and transfer case.


The most pressing question regarding when to switch a new engine to synthetic oil is the “break-in” process. The concern was always that synthetic oil was so “slippery” that the piston rings would not seat properly and oil consumption would plague the engine going forward.

While break-in may have been a legitimate concern in years past, today’s manufacturing processes are so precise and tolerances are so close that a break-in period is no longer necessary. AMSOIL Technical Services tells us that new car engines are essentially broken-in before they are shipped.

So can synthetic oil be installed at mile one? Yes it can and you will not have oil consumption issues because of it. In fact some car engines come filled with synthetic oil from the factory.  But, we still suggest keeping that factory fill in the engine for a while before switching over to a high quality synthetic engine oil. The reason being is there may be tiny filings, manufacturing by-products and silicon leaching out of the gaskets that will contaminate this new oil. We suggest that it is better to let the original oil and filter get these contaminants out of the way before switching over. A period of 800 to 1500 miles (1200 to 2400 kilometers) is a safe period to make the earliest change, in our opinion.

One can certainly switch a bit sooner or later, as this is a rough estimate. We do see some auto  manufacturers and dealers suggest waiting until the first scheduled oil change which can be up to 10,000 miles (16,000 kilometers) and more. While this may not be the end of the world, we do not feel comfortable with having the factory fill and all of the accompanying contaminants in place for that long.


There is a sound contention that manual or automatic transmissions do not require a break-in period at all. Also as there is no combustion, one could argue that transmission fluid has a much easier life than engine oil. That being said, a switch to synthetic transmission fluid is very beneficial as it is much more resistant to extreme heat, cold temperatures and heavy loads than factory conventional fluids. That is why many new car and truck owners are anxious to flush out the conventional factory fill at the earliest opportunity. When should this occur?

Most car manufacturers will suggest changing the fluid at the regularly scheduled service interval, but this can take years to reach. Due to gasket residue, clutch material and gear filings, we are not comfortable with waiting that long. An interval of around 15,000 miles (24,000 kilometers) is a completely arbitrary milestone, but that is the point that we feel it’s time to switch to synthetic transmission fluid. By that point the factory fill has done it’s job of flushing out any contaminants that come with new transmissions.


Many of today’s differentials in trucks and SUV’s come equipped with synthetic gear lube right from the factory. So does it need to be changed? Evidence suggests that it should be changed and much earlier than you might think. Differentials do indeed go through a break-in process and see the appearance of significant wear metals in the early going. Testing shows that the majority of these wear metals show up in the first 5000 miles (8000 kilometers) of the differential’s life.

Therefore, we suggest dropping the factory fill at that point and switching to an elite synthetic gear lube like AMSOIL Severe Gear. You are then set for a long time.

Source: Differential Oil Change After Initial Break-in (123k PDF)

Transfer Case

Transfer cases can benefit greatly from a synthetic fluid, including improved wear control, better engagement in cold weather and resistance to extreme heat.  There are many types of four-wheel drive systems (full-time, part-time, automatic) and the depending on the vehicle’s usage, the amount of load on the transfer case can vary quite drastically.

These variables leaves when to drop the factory fill wide open. As a transfer case generates wear metals derived from gears and chains, we change out the fluid when we change the rear differential oil for the first time. Do keep in mind that we live in the snow belt, so our 4WD system sees regular service. You may want to wait for a full year, if your 4WD system doesn’t see much action.


As you do not change transmission, differential and transfer case fluids very often, why not put the best available products into service? We can offer the superb AMSOIL Synthetic Oil line at wholesale shipped from Canada or the US. Request a price list using the form at the top-right of this page or call us at 1-800-748-5781.

To find the correct AMSOIL Synthetic Oil for each component of your vehicle, use our helpful Online Application Guide.