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the main difference is basically the hot viscosity, 5w40 is thicker than 5w30 when the engine is at operating temperature. the 5w at the start is the same for both (cold viscosity), it's the 30 vs 40 that makes the difference. for a diesel it matters because these engines often run at high temperatures and put more stress on the oil.
5w40 will protect better if your engine is worn, if you regularly haul heavy loads or if you drive a lot in the mountains. on the other hand it uses a bit more fuel and it's thicker at startup. 5w30 is lighter, better for fuel economy and it flows better in cold weather. me after 17 years working with diesel vans i'd say just check what your manufacturer recommends in the manual, it's not marketing it's made for your engine specifically.
honestly if 5w30 is recommended just use that, you're not gonna gain much by going up in viscosity and you'll just waste fuel for nothing. change your oil regularly that's what really matters with a diesel.
Like I said above, it's really all about hot viscosity. The 40 is thicker than the 30 when your engine's running at its normal temperature, that's clear. But anyway, before you decide, you gotta check what your car's manufacturer recommends - that's really the starting point. Why am I telling you this? Because going too thick in viscosity for an engine that needs 30, it can mess with fuel consumption and even the engine's performance over the long term. It's not a disaster, but it's still something to consider.
The thing is, if your diesel has a bit of play in the pistons or if you're in a really cold climate, sometimes people go for 40 for extra protection, but again it's gotta be compatible with your engine. What model do you have exactly? And how long have you been using it? That would help me tell you better if switching to 40 would change anything for you. Because honestly, if the manufacturer says 30, generally that's what you need to use.
maxime_dubois asker
Peugeot 807, 2012, 170,000 km. The manufacturer recommends 5w30, but the engine burns a bit of oil in summer. Thanks for the advice.
Yeah so like I mentioned above, the main difference is the viscosity when it's hot. The 5w40 is gonna stay thicker when your engine's running at normal temperature, while the 5w30 is gonna be more fluid. So if you use 5w40, you're gonna get better protection of your parts and better sealing of the joints, but it's also gonna consume a tiny bit more energy since it's thicker to circulate.
For a diesel, it really depends on what your manufacturer recommends. Check your maintenance booklet or on your oil filler cap, usually it's specified there. If your diesel already uses a bit of oil or if you've got an older engine, the 5w40 can be interesting. But if your engine is new or if it runs well with 5w30, no need to change, it's just not gonna give you much except maybe a slight increase in fuel consumption. The important thing is really to stick to what the manufacturer says rather than trying random stuff.
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