NOTE: The advice below is relevant for 1989-2005 NA and NB MX5s, the concepts can be used in any vehicle but you should always work back from the manufacturer’s specifications and real world testing.
For a little more context, please do read this post first; https://bofiracing.co.uk/blog/heres-why-amsoil-is-the-best-oil-for-your-mx5/
Assuming all oil is created equal and all weights between different brands perform the same (spoiler, they don’t) – we’ll just talk through how we choose the right oil weight for our MX5s.
A couple points to keep in mind, is that the bigger the difference between the cold weights (10W) and the hot weights (40) often results in lower oil pressure when everything is up to temperature, they generally can’t take as high a temperature as their higher cold weighted counterparts. The oil manufacturers and engineers do work magic to make oil flow like water when it’s cold yet remain thick enough when hot to provide adequate engine protection.
If 10w is what Mazda recommend for the UK climate all year round, you generally don’t need to go lower than a 10w, you certainly can with a higher quality oil as this will give your mx5 an easier time starting and often provide a little better initial startup protection – but it is a delicate balance between a good starting protection and how the oil performs under extreme stress.
Into the detail
Let’s talk about the OEM / Mazda recommended oil weight – at the time the recommendation was for a semi-synthetic oil and for the temperature range in the UK is for 10w-40, it provides enough cold performance to deal with our relatively mild winters while at the same time offering a thick enough film to deal with spirited driving.
Case closed. Well, it’s really not that simple when we start modifying and doing motorsport or track days with these cars. The biggest consideration is an oil that has a high film strength, this isn’t advertised on the front of the bottle and it can’t fully be inferred by the oil weight.
Let’s talk about the contributing factors;
Naturally Aspirated – Spirted Street / All Year Driver
Here, we want an oil that will put up with daily driving, cold starts and still remain thick enough in the hottest of summers. We would be happy to see either a 5w-30, 10w-30, 5w-40 or 10w-40. Consider the 5ws if you’re using it in Dec-Feb, consider the 10w if you’re putting it through any form of track day or alto solo.
The higher weight of the 40 will hold pressure far better when hot, on a 1.6 you’ll know the cars unhappy if the lifters start tapping away when hot, typical on a 30w after a track session.
Naturally Aspirated – Trackday no Oil Cooler
Here we’re assuming you’ve got a passion for speed and don’t let off the gas, without an oil cooler you could see temps up to 135c in the oil – here’s looking at you Salon Motorsport – the takeaway here is really to use an oil cooler but if that wasn’t an option we’d like to see a really good 10w40 at a minimum and potentially up to a 5w50, 10w50 or even a 15w50 for the hottest of climates.
Naturally Aspirated – Trackday with Oil Cooler
Now you’ve upgraded and moved to a thermostatically controlled oil cooler running at least an 82c thermostat on a 16 row cooler, in a fairly well-positioned space we are less forced to go with the higher weight oils to compensate for pressure loss as the oil starts to get hot.
We’d be happy with a quality 5w-30, 10w-30 or up to a 10w-40.
Supercharged – Trackday no Oil Cooler
Firstly, go an get an oil cooler, you’re putting a bunch more cylinder pressure into the engine which is forcing those pistons down pushing that oil film apart, as everything starts to get hot that oil film is going to be highly stressed and will have a hard time keeping everything together.
With that in mind, we’d want to see a 5w-50 if it’s used in winter, or a 10w50 oil, if you have an oil temperature sensor then feel free to experiment with a 10w-40 but we would fully expect the temps to get above 120c when driven hard and the pressure to start to fall off.
Supercharged – Trackday with Oil Cooler
Now you’ve upgraded the same rules apply, we don’t have to use the oil weight as so much of a crutch to keep the oil pressure higher when there is heat in the system.
We’d be happy with a 5w-40, 10w-40 or even a 5w-50 still. We like to see a lower initial weight on a supercharger for cold/initial startups due to the additional load on the snout of the crank, which in turn creates an additional load to the bearings.
Turbocharged – Trackday no Oil Cooler
Turbocharged cars put even more heat into the engine, through the added load of the cooling system flowing through the turbocharger as well as the oil itself flowing through the turbo and being heated, consider that this same oil needs to lubricate and function within an incredibly hot turbocharger, a quality oil that doesn’t break down and can handle the heat is key here, especially if you’re not helping it out with an oil cooler.
We’d want to see a 10w50, 15w50 or potentially up to a 10w60 if it was high quality. We have run engines on 10w 40 on hot track days but have experienced pressure drop to concerning levels when fully hot.
Turbocharged – Trackday with Oil Cooler
You’ll be needing a thermostatic oil cooler of at least 16 rows for this recommendation to apply, the goal here is to keep the oil temps around 100c while in operation, much more than 110c and some oils start to struggle to cope.
With that in mind and provided you do have a way of measuring your oil temp and pressure, we’d recommend a 10w40 or a 10w 50 – we do run 15w-50 in our turbo 1.6 because we want the film strength offered by the dominator package.
It’s long been a recommendation from the forums to go with a 5w-40 or a 10w-40 often arbitrarily, we’ve tested tonnes of different brands in the same weight, they perform very differently to each other. What we’ve put forward above, you pretty much can’t go wrong with the oils recommended, pick the one you like the price or the packaging of the best.
The AMSOil boasts a higher life and we can certainly believe that budgeting multiple oil changes throughout the track day season is expensive and we’d rather run one lot of high-quality dependable oil and then change it when we start to see the pressures falling with heat, probably 6-10k miles in on a turbo car.
We know from experience that you can run a lower quality oil for extended periods and not blow the engine, but when it starts to get into the realms of built engines or even just engines with turbochargers attached an engine failure can be far more costly, we also don’t want to be the guys that ruin the afternoon of a track session leaving a trail of oil through the new hole in the block.
In a daily driver situation, it is more important to change the oil often with reasonable quality oil than it is to just run high quality to the end of its lifecycle, but for our needs, it has to stand up to some real abuse.
One of the main takeaways here is an oil cooler really lets you get away with a lower weight, which does have its side benefits of freeing up some marginal horsepower numbers and knowing your oil temperature and pressures really is key to keeping everything spinning in harmony.