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Hi, I have 68 dart where i am new owning this car. I need to do an oil change and had purchased 5w-20. This will be my first time to do this. I want to know if this is the correct viscosity? Sorry for my ignorance. I read a good article link https://motoringcrunch.com/reviews/0w20vs5w20/ for 5w-20 and seems to be good for this car.
 

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Oil

I would run a minimum of 10W30 or 10W40 oil in your Dart. The 5W20 is not heavy enough, & I'd run a ZDDP zinc additive since its a flat tappet cam. How many miles do you have on the car & what do you want to do with it? i.e. street cruiser, highway cruiser, race car?
 

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I agree. 10-30 winter 10-40 summer Most oils today don't have enough zinc for your stock cam shaft so yes to zinc additive.
 

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I have been adding a bottle of stp which says it has zinc in it to valvoline 10-30, cannot find the racing valvoline I used to use.
 

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What oil weight ???

Poor or no zinc is a big problem for older cars - I would an adder to your oil to protect the cam / tappets , and 10w30 oil. Some people like to use diesel oils like Shell T3 or T4 t that I use in my race engine.
 

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I agree with what he said about zinc. Most 10-30 winter 10-40 summer oils do not really have enough zinc for your stock camshaft.
 

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I have used Brad Penn for many years on my flat tappets, now it is called Penn-Grade 1, it is available in 10-30, 20-50, 50, and 70. Even this fine oil now has slightly reduced its zddp content so I now add 2 oz of 99000 to each oil change. If I cannot obtain Penn-Grade I use Shell 15-40 Rotella-T diesel oil spiked with a 4 oz bottle of Howard's Cams 99000 ZDDP additive which is only about $7.00 and puts the worries to rest. Regardless of the oil choice, at around 1500 miles on an oil change, or if I need to add oil along the way, I always add 2 oz of the 99000 because the zinc and phosphorus do wear out and decay over time.
Beware! Other than Penn-Grade 1 none of the common "big" name racing oils available today have sufficient zddp for high spring rate flat tappet engines. Ignore all the high and mighty claims printed on the jug, unless you add the zddp to them they will all eventually cost you a camshaft, or scuff up your skirts.
I use the same oil/s on my rollers as well out of deference for valve stem tips push rod ends and piston skirts. One cannot be too safe, as high concentration zddp lubricants cost essentially nothing in comparison to the extensive damage wrought in their absence.
Viscosity guidelines: If your crank clearances are .001"-.0015" use 10-30. If .0015"-.002" use 10-40 or 15-40. If .0025"-.003" use 20-50. When in doubt use the lighter weight oil as flow is your friend when it comes to engine oiling. Save your 0-20 or 5-20 oils for your 1993 or newer minivan or whatever.
Steve
 
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