Lowering viscosity of engine oil
 

Hi.

I have an 8 year-old Mazda3 2.0. Is it safe to lower the oil viscosity from 5W-50 to 40? I'm currently using fully synthetic oil.

Seriously? Your car is supposed to be using 5W-20 if you're in the U.S. and Canada, and 5W-30 everywhere else in the world; why on earth are you using 5W-50?

It's a used car and I don't know what oil viscosity the previous owner used. I asked 2 different mechanics (both are owners of their respective shops) who are into drag race and they both said it was ok to use 5W-50. They even own those 80s car and are still running. I trusted them on this one. I don't have any performance issue by the way. Everything seems ok. At that time, there was no 5W-30 available in the shop too. I'm not a fan of Mazda shops because they charge high and price is so expensive (twice of what I can get from others). They (not just Mazda) let trainees work on your vehicle too (why I would I want that) and they don't let you see the work. I'm in South East Asia so it's hot and humid here without snow. I wonder if it's safe to lower viscosity to 10W-40. If it's safe, I'd even go for 5W-30.

Sorry, anybody who tells you it is okay to use anything heavier than 5W-30 in your engine is an absolute idiot. Yes, cars from the 1980s could easily be designed to run on 10W-40, and as those engines age, a 5W-50 wouldn't be an issue, but in your 2008 Mazda3, nope, sorry, they're dead wrong.

Now I'm worried. :(
Is it still safe to use a lower viscosity? I've been using 5w-50 for around 1.5 years now. Only around 6000 Km run since last oil change though.

I don't understand, why would it be unsafe to use the oil recommended by Mazda?

5w30 or 10w30 would be best suited to your vehicle.

CASTROL EDGE 5W-30 A3/B4
Castrol EDGE 5W-30 A3/B4 is a fully synthetic, SAE 5W-30, engine oil. It gives you the confidence to demand maximum engine performance from today's highly tuned four cylinder and latest technology engines, operating with tighter tolerances, requiring lower viscosity and stronger oils for variable service intervals. Castrol EDGE 5W-30 A3/B4 is proven in tests to deliver outstanding oil strength.

I have an old car. I read somewhere that since I have used a thicker oil and an old car, there are spaces in the internal components and that thin oil is not ideal. I've read that their cars start to burn oil or leaked.

I checked YouTube before replying to this post and in the comments section some people are saying no if I have used a thicker oil.. But they're not talking about Mazda so I just want to make sure and get inputs from Mazda owners before I change oil.

Link:

What :people say" is utterly and completely irrelevant, what the manufacturer says IS relevant. Use 5W-30 and be done with it. As for the brand of oil; who cares? Oil is like religion, use whichever brand you worship, change it per the manufacturer's schedule, and it will make no difference in how long your engine lasts.