YMMV: Castrol GTX High Mileage Synthetic Blend 5 Quart in various grade $14.95

fandrew7

Apr 28, 2026 08:42 PM

222 Posts

Joined Mar 2015

fandrew7Apr 28, 2026 08:42 PM

222 Posts

This is a good deal, but various weights are available on Amazon for this same price. For the five dollar rebate savings I'd rather not have to deal with advanced autos incredibly inaccurate inventory and the gas to go get it.

brotherhpj41

Apr 28, 2026 08:55 PM

3,860 Posts

Joined Oct 2010

brotherhpj41Apr 28, 2026 08:55 PM

3,860 Posts

Always wondered:
Synthetic blend motor oil generally contains between 10% and 25% synthetic base oil mixed with conventional oil, though some blends may vary from as little as 1% to as high as 60%. There is no industry-wide standard defining the exact percentage required, allowing manufacturers to decide the ratio, often keeping it closer to the lower end for cost reasons

2

fandrew7

Apr 28, 2026 11:33 PM

222 Posts

Joined Mar 2015

fandrew7Apr 28, 2026 11:33 PM

222 Posts

Our community has rated this post as helpful. If you agree, why not thank fandrew7

Quote from brotherhpj41

:

Always wondered:
Synthetic blend motor oil generally contains between 10% and 25% synthetic base oil mixed with conventional oil, though some blends may vary from as little as 1% to as high as 60%. There is no industry-wide standard defining the exact percentage required, allowing manufacturers to decide the ratio, often keeping it closer to the lower end for cost reasons

While full synthetic is a better option on any engine that doesn't have leaky seals, semi synthetic will often leak less than full synthetic if you have old or dry seals. Conventional will leak even less. It's not uncommon for an older engine like Jeep's 4.0 to visibly drip full synthetic but be completely dry with conventional oils. Every engine is different and the type of oil (which detergent packs are used in the blend) matters. If you have a small leak/drip try switching to something like white bottle Castrol (conventional) or a semi-synthetic like gtx high mileage if you need the thinner viscosity.
Source: mechanic for ~25 years

3

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ZV3

Apr 30, 2026 01:56 AM

1,357 Posts

Joined Dec 2008

ZV3Apr 30, 2026 01:56 AM

1,357 Posts

Not sure who a semi-synthetic is marketed to, somebody who ALMOST wants the best? Someone who wants to save a few bucks over full synthetic but not reap any benefits? Just go full synthetic for 3 or 4 bucks more.

1

WthaHatchet

Apr 30, 2026 04:41 PM

255 Posts

Joined Nov 2022

WthaHatchetApr 30, 2026 04:41 PM

255 Posts

Quote from fandrew7

:

This is a good deal, but various weights are available on Amazon for this same price. For the five dollar rebate savings I'd rather not have to deal with advanced autos incredibly inaccurate inventory and the gas to go get it.

If you order from them, Advance auto, make sure you check your email for either order ready for pickup or canceled before going. Will stop you from going for nothing. Just fyi. They aren't always inaccurate. Depends on the individual stores.

WthaHatchet

Apr 30, 2026 04:44 PM

255 Posts

Joined Nov 2022

WthaHatchetApr 30, 2026 04:44 PM

255 Posts

Quote from fandrew7

:

While full synthetic is a better option on any engine that doesn't have leaky seals, semi synthetic will often leak less than full synthetic if you have old or dry seals. Conventional will leak even less. It's not uncommon for an older engine like Jeep's 4.0 to visibly drip full synthetic but be completely dry with conventional oils. Every engine is different and the type of oil (which detergent packs are used in the blend) matters. If you have a small leak/drip try switching to something like white bottle Castrol (conventional) or a semi-synthetic like gtx high mileage if you need the thinner viscosity. Source: mechanic for ~25 years

If you have and older engine the high mileage synthetic has additives that make the seals swell more. So leaking can be the same depending on if you used the high mileage or not. Id do agree though regular motor oil is thicker for some reason even at the same weight it seems.

asuka

May 01, 2026 02:52 AM

2,537 Posts

Joined Oct 2008

asukaMay 01, 2026 02:52 AM

2,537 Posts

As usual, old engine? Great.

Turbo? Supercharger? Timing chain? Get an API SQ full synthetic.

1

mouthforwar

May 01, 2026 06:17 AM

1,104 Posts

Joined Apr 2014

mouthforwarMay 01, 2026 06:17 AM

1,104 Posts

Amazon has 10W-30 and 5W-20 for the same price (cheaper with S&S) if those suits your needs: https://www.amazon.com/Castrol-03...43WE/?th=1

https://www.amazon.com/Castrol-03...LS64/?th=1

Last edited by mouthforwar April 30, 2026 at 11:19 PM.

dkelsey56436

May 03, 2026 01:48 PM

14 Posts

Joined Jul 2017

dkelsey56436May 03, 2026 01:48 PM

14 Posts

Quote from fandrew7

:

While full synthetic is a better option on any engine that doesn't have leaky seals, semi synthetic will often leak less than full synthetic if you have old or dry seals. Conventional will leak even less. It's not uncommon for an older engine like Jeep's 4.0 to visibly drip full synthetic but be completely dry with conventional oils. Every engine is different and the type of oil (which detergent packs are used in the blend) matters. If you have a small leak/drip try switching to something like white bottle Castrol (conventional) or a semi-synthetic like gtx high mileage if you need the thinner viscosity. Source: mechanic for ~25 years

My Northstar just said "hold my cosmo"

YMMV: Castrol GTX High Mileage Synthetic Blend 5 Quart in various grade $14.95

$15

$30

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