I have a ‘66 Valiant Signet 225. The previous owner said they ran 20W50 oil in the summer & 10W30 in the winter. Is this correct, or are there other oil types preferred for the older models?
Thanks!

There are as many opinions on oil as there are people in the world. My two cents: use what the manufacturer suggested in the owner's manual. Personally, I've used synthetic since the early 80s, but always in the grade the manufacturer suggests. They know a thing or two about the engine they built.

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If it was easy, everybody would be doing it.

5w-30 or 10W-30 in any brand with the API sunburst logo. Sythentic is a nice upgrade. Don't forget a good oil filter. Whatever you do, don't run a Fram filter.

10W-30 is good year-round unless the engine is on it's last legs. I like 15W-40 in high mileage engines. Fram makes some good filters, but not their cheapest line, Extra Guard.

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Joshua

I expect your 66 owners manual to be very similar to my 65 owners manual. Chrysler only recommended 20W-40 as the thickest oil for hot weather. A readily available and very low cost modern equivalent is 15W-40. See Chrysler Oil Recommendations.

I'm still using the original oil pump in my car but have upgraded to the 75 psi hemi oil spring. On the hottest days here in Ontario, I can get 70-75 psi @ 2500 RPM after hours on the highway using 10W-30. That means that my bearings are always getting the full flow of the oil pump. With a stock 45-55 psi oil spring, a significant amount of any 30-grade (0W-30, 5W-30, 10W-30) oil and relatively more 40-grade (5W-40, 10W-40, 15W-40) will relieve back to the sump. See Engine Wear.

For flat-tappet older engines without catalytic converters, an API Starburst oil isn't required because Starburst Oils limit phosphorus (the anitiwear component of ZDDP) to 600-800 ppm. Heavy Duty Engine Oils (API CJ-4 & CK-4) have their phosphorus contents limited to 1200 ppm. Racing oils (like Valvoline VR-1 10W-30) have lots of ZDDP but less detergents. See Corvair Oil Guide.

I'm using Petro Canada Duron SHP 10W-30 in my car. Shell Rotella T4 10W-30 is another popular high ZDDP oil for older engines. For stock valve trains, Shell Rotella T6 5W-30 is an excellent synthetic oil.

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1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech

I started using this stuff about 6 years ago: http://www.getg.com/G-OIL/bio_full_synt ... or_oil.php

It's a bit spendy, but after 200K there was zero goop or buildup under the valve cover. Top of the engine looked clean as a whistle. It's a decent Group V oil. I like that it's biodegradeable.

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If it was easy, everybody would be doing it.