Hi, I have a 1988 K5 ,5.7. I have no workshop manual or owners manual. The oil filler cap states I should use 10w/30 engine oil. For some unknown reason it is getting difficult to source 10w/30's here in the UK. Am I OK to use 5w/30's as I already have some in my garage ? I will be doing an oil change rather than just topping up the existing oil.
Thanks.

 

I don't think it should be a problem. If you have access to Castrol 5W30 that might be a good bet because, IIRC, Castrol tends to run a little on the high side of the viscosity range for a given weight.

 

What are your year round temps, Winter/Spring/Summer/Fall?

 

I don't think that running 5w30 in it will hurt anything. I run 5w30 in my Bonneville which also calls for 10w30 and have not had a single problem with it

 

Here in my part of the UK the temp ranges from about 23F(-5C) in the Winter to 82F(28C) in the Summer.

 

You should not have much trouble with 5w30.

 

I did the oil change with Mobil 1 fully synth and K+N Oil filter.
Its running great. Since changing the oil I've bought a Haynes manual which recommends either the 10w30 or 5w30.
One thing I did notice which I need to look at next is an oil leak from the back of the engine somewhere which seems to run down on to the side of the oil filter and sump. Any ideas ?
Thanks.

 

Could possibly be a valve cover gasket, oil pressure sending unit stand pipe, the lower intake manifold seal at the back of the block or even the distributor base gasket. Since it is coming down on the driver side of the block, I would look at the driver side valve cover gasket and the oil pressure sending unit stand pipe.