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What really matters most is what your manufacturer recommends, because using the wrong viscosity can seriously mess up your warranty. Check your maintenance manual, it's spelled out plain and clear. Yeah, the guys before explained the hot viscosity thing to me, and that's correct, but there's a detail people often forget: 5W40 uses slightly more fuel in winter because the engine has to work a bit harder to circulate thicker oil. It's not a big deal though, we're talking a few percent, but it adds up over time.
The other thing: if you mostly drive in town or do short trips, the 30 will do the job better since your engine will stay at a less stable temperature. On the other hand, if you're doing long motorway drives with an engine that gets hot and you're stressed about hauling something heavy, the 40 is more cautious. As for the 5W at the start, that's the fluidity when cold, and honestly both do the job. You really need to focus on that final number.
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.
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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