Join Date: Jul 2004
Location: Northern Virginia
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What have you been using in it thus far - assuming 5w-30? Stick with it then, the old thoughts of 5w-30 being too thin are completely baseless. 5w-30 has the same viscosity as 10w-30 when the engine reaches operating temperatures - hence the 30 weight. At lower temperatures (doesn't have to be subzero - even cold starts in general), the 5w-30 will flow faster than 10w-30 - so if you are talking about engine wear, which most wear starts on initial startups, 5w-30 is the better choice, protection wise.
10w-40 is a little heavy to run in the car. Unless you plan on tracking/racing the car exclusively (high heat, extreme pressures, etc.). For a daily driver in a wide variety of temperature ranges and still wanting to maintain good fuel economy - stick with the manufacturers recommendation of 5w-30 motor oils.
The formulation and design of current motor oils in terms of engine protection, deposit control, and viscosity/temperature stability - are a quantum leap ahead of oils made in the past several decades. Sounds like the salesperson is using some old school of thought on engines that had much less control and larger clearances than what the 1ZZ-FE is running.
I personally would run only a 5w-30 or 0w-30 in my cars. 0w-30 for the winter time (temps can occasionally dip below zero on my area or when I'm on travel out of state). On occasion, I will run 10w-30, but usually in cases where I ran out of my stock of oil and can't find any 5w-30 or 0w-30. The car will run fine on the 30-weights oils, but I tend to loose 5-10% in fuel economy if I run the 10w-30.
Bottom line, since your car is running fine and doesn't consume oil - just stick to what you've been doing. So far you've treated the engine right, and it has treated you well in return - no reason to change up what you are doing.
2002 Toyota Corolla S, 1.8L 1ZZ-FE VVT-i
2003 Toyota Matrix XRS, 1.8L 2ZZ-GE, VVTL-i (RIP)
2009 Toyota Matrix XRS, 2.4L 2AZ-FE VVT-i
2009 Toyota RAV4 Limited, 2.5L 2AR-FE Dual VVT-i