98 riv 10w30, 04 regal GS 5w30

Obviously both sc 3800 but why the different oil recommendations? Taken right from the oil fill cap 98 riv says 10w30 04 gs says 5w30..

Dan B wrote:

Obviously both sc 3800 but why the different oil recommendations? Taken right from the oil fill cap 98 riv says 10w30 04 gs says 5w30..

Don't overthink it - and maybe talk to a tech or service writer at your local GM dealer.  They should tell you that GM changed the spec from 10w30 to 5w30 some years ago and will use that oil as the fill on your Riv is you bring it in for an oil change. You will also see details about use of 5w30 in your owner's manual - it has always been listed as an optional oil for the stated temperature range.

I use 5w30 in the winter, 10W30 in the summer.

It's a 3800, it'll take whatever oil you throw at it. More important part is being consistent with brand, different companies might use different detergents that could clear up the gunk in your system. Which sounds good, until you start leaking by the pint every week