I have recently had a oil fired boiler fitted and have noticed that some of the external joints (compression type fittings) that come from the oil tank are starting to weep. I noticed when the plumber fitted these he used some kind of red paste on the joints, is this the correct method?
Often compression joints used without any sealant will weep when used with oil.
Since the "olden" days, I have used a product called "Red Hermetite" which is a non setting sealant that is resistant to temperature and oils. This product or similar was used everywhere from car engines to oil installations.
I believe it is no longer available, but a similar product called Red Hylotyte can be found in most garages and car accessory shops.
Generally, I would say he has used the correct product, but of course that doesn't mean joints won't leak. Oil is a beast, because it doesn't evaporate, even the smallest weep from a joint will spread and appear worse than it actually is.
If the joint is leaking maybe a quick nip up will solve the problem, or perhaps some more sealant is required.