I drive a GMC(its work, i have too) topkick at work and we use 15w-40 in the 366ci engines. my question is can i use the 15w-40 in my bronco that has a 300 I6? is it too heavy to put in my truck? i never cared much about weights of motor oil and dont know that much about it! thanx for any help:-X11

 

My '81 owners manual lists these oil viscosity reccomendations:

Single grade:
10w
20w20
30
40

Multi-grade:
5w20
5w30
10w30
10w40
10w50
20w40
20w50

Of course there are appropriate temperature guidelines to accompany these oil reccomendations.

While it doesn't specifically mention 15w40, it does not prohibit it either. I think at the time of printing (1981) 15w40 was just not readily available to most motorists because it was primarily intended as a commercial diesel engine oil.

As long as the oil meets the API requirements for gasoline engines too, it should be an acceptable oil to use. I have used 15w40 oil of several brands in both gasoline and diesel engines, turbocharged and naturally aspirated. Most brands of this weight are high quality, but it's not recommended by the manufacturer's of the newer gasoline engines. Because it is a heavier weight, it does not carry the API "energy saving" symbol.

 

I use 15w40 all year around in my 302. I live in PA so we have temps in the 20's up to the 90's. In winter it starts a little harder not much to matter. I don't trust a 10w or a 5w for the summer, too thin and it is a pain to have to switch oils twice a year so I use 15w40, a happy medium!