mpg on a '75 Clamshell-What should I expect?

I realize this is a seven year old post, but I have to say: Yes, 20w50 is not most ideal for MPGs and it causes more engine wear than a 10w50 would. Ideally for maximum MPGs from oil as far as viscosity is concerned, use the thinnest oil you can find which still gives you proper oil pressure at operating temperature.

 

So if I were to use 20w50 motor oil how much sooner will my engine wear out and how bad will my fuel mileage be compared to using 10-w50 oil?
For an older engine you need to be more concerned about zink and ZDDP. Split the difference and use 15w40 Rotella T diesel motor oil. Dello 400 15-w40 is also good.
Happy motoring. For safety add one pint of STP additive. Your mileage may vary!

 

I always used 15-40 diesel oil in mine. I use lucas I used to use STP but I like lucas a lot better. In my wagon tho the internals were all new and really had no need for the lucas.

 

Son uses different Lucas oils in about everything as an additive. The way he tears things up I have no idea if they work!
He could ruin a brick with a feather.

 

I don't know how much faster your engine would wear out, but the thing is that when reading the oil bottle the first number (0/5/10/15/20w) says what viscosity it is when you start the car and the second one says its viscosity at operating temperature. The lower the number on the left, the more it; lubricates when you first start the car, and the higher the MPGs are at startup. Shell T6 5w40 is what I'm using in my 1984 Caprice (5.0 Gas motor) now. It's a synthetic oil, and I haven't had an issue with it. Granted the previous oil (Pennzoil High Mileage 5w30 + STP) was put in under 2,000 miles before, so it was already a bit cleaner. I'd like to use a 0w30 in the Caprice next year.

There is also the subject of oil shearing if it's a multi-viscosity oil, but this isn't the 1950s or 1970s here. Research is vital and I don't know what to say on the subject of oils shearing. The subject of synthetic oil "causing an engine a rebuild" is also somewhat of a subject, but the synthetics swelling seals is just decades old subject and it shouldn't effect seals these days. Moreso, if your engine's sludged then switching to synthetic could cause sludge to break away and clog oil passages. But, for my Caprice I've already used a synthetic before and it didn't cause an issue and it's not doing it now so I'm confident in using a 0w30.

 

BTW, those oil thickener additives - STP and Lucas in particular, maybe others - really shouldn't be used in a new engine if it calls for 0w/5w20 because they thicken the oil way too much. If it's an old worn engine that leaks and burns oil and calls for 10w30 or thicker, maybe. But otherwise, wouldn't it void the warranty?