Stick with what your choice is...many caps say 5W-20 because of EPA standards. People at Wally World have no business telling you you're wrong.

 

I'm not sure why your engine "doesn't like it" and what exactly that implies. 🤔
A 30 weight oil won't hurt but at 102,000 miles is it really necessary?

It is good that you're maintaining proper oil change intervals and taking care of your car. 👍
I personally don't like either the Castro or the Valvoline but if your having good results with the Castro's high mileage then continue using it. 👍

 

He claimed my engine will start knocking around and stuff if I use 5w-30.

Just because he knows how to unscrew a drain plug and filter doesn't mean he knows much about engine lubrication.

 

refuse to use my oil 5w-30 by Castrol.

That Castrol 5w30 Synthetic is great Oil technition (Ford Boss) uses Castrol GTX 5w30 synthetic less burn off over Mobile1 wally mechanic wrong.!!

He claims I should follow what's on the oil cap which is 5w-20,

You can dont always mean you have too.. especially if the vehicle has some higher miles a little bump in viscosity wont hurt engine

He claimed my engine will start knocking around and stuff if I use 5w-30.

totally bogus that's why? wally mechanic works at wally world.

 

Has anyone been told that using 5w-30 instead of 5w-20 will cause knocking in the engine?

It's possible they only have 5w-20 in bulk (cheaper)...if you request something different it will cost them more.

 

Why would it cost them more to use customer supplied oil sitting on front seat ?

We charge labor, and filter if customer supply their own oil

 

Wow, where are you pulling that out of?

 

Stated that mechanic refused to use his own Castrol 5W-30.

 

The guy has a point. 5w30 is higher viscosity and will hurt fuel mileage. Instead of 25 mpg, it may drop to 24.99 mpg. If in a climate with sub-freezing temperature it may drop to 24.88 until warmed up.

When I was a teenager driving "older" cars that used oil, I'd put in 40 weight to help.

 

OK, when in doubt, go to the owners manual (see below) As the owner of the highest mileage (734,000) Accent in the forum, I pay attention to my oil,I pulled my owners manual out of the glovebox and took this picture. Unless your car spends time below 0 F, you should be using 10/30. I put 10/40 in mine now and have been since about 400k.

 

Correct, use what is recommended in the users manual.

As for the 5W-20 versus 5W-30 oil, it does have a higher viscosity at the higher temperature rating. Remember, all the multi-grade oils are just a chemically formulated oil that has a defined tested viscosity. As the temperature increases, the oil gets thinner, or lower weight. the 5W-20 has the same basic weight as the 5W-30, but the 5W-30 oil has a different type or amount of an oil additive so it "appears" to be a 30 weight oil at temperature verses the 20 "appearing" like a 20 weight at temperature. It just "appear: to be thicker. In reality, the actual viscosity at the higher temperature is much less than the base oil, even in oils like 5w-50. It just does not "thin out" as fast as "without" the additive. But the higher number means that it acts like a "normal" oil weight of that viscosity. the 5W-20 has the same viscosity at high temperature as the "standard" 20 weight oil.

Anyone remember STP? That was reported as performing the same function; keeping the oil from becoming too low a viscosity at temperature.